Raid data migration through stripe swapping

A system and method for improving storage system operation is disclosed. A storage system includes a first tier with high-performance redundancy and a second tier with capacity efficient redundancy. The first tier and the second tier are built from the same storage devices in a storage pool so each storage device includes both the first and second tiers. The storage system stores write data initially to the first tier. When demand for the data falls below a threshold, the storage system migrates the write data to the second tier. This is done by changing the mapping of underlying physical locations on the storage devices where the write data is stored so that the underlying physical locations are logically associated with the second tier instead of the first tier. After remapping, the storage system also computes parity information for the migrated write data and stores it in the second tier.

TECHNICAL FIELD

The present description relates to data storage systems, and more specifically, to a technique for the migration, with low overhead, of data between two different storage tiers of different performance and redundancy levels/types.

BACKGROUND

A storage volume is a grouping of data of any arbitrary size that is presented to a user as a single, unitary storage area regardless of the number of storage devices the volume actually spans. Typically, a storage volume utilizes some form of data redundancy, such as by being provisioned from a redundant array of independent disks (RAID) or a disk pool (organized by a RAID type). Some storage systems utilize multiple storage volumes, for example of the same or different data redundancy levels. Different storage volumes may have different data redundancy levels to take advantage of the different performance levels at a variety of workloads.

For example, some storage systems may have a first level with one or more storage volumes that have a first redundancy level. This first redundancy level may be a RAID level, such as 0, 1, or 10 as some examples, that provides a faster response time for small input/output (I/O). The storage systems may have a second level with one or more storage volumes that have a second redundancy level. The second redundancy level may be a RAID level, such as 5 or 6, that provides better capacity utilization (e.g., over RAID 1 or 10) and/or better device failure tolerance. In such tiered systems (those with at least two different storage volumes having different redundancy levels), the first, faster tier may have less capacity in comparison to the second, slower tier which provides better capacity utilization.

As a result, data stored in the first tier may occasionally be moved to the second tier to make room in the first tier for data that is more in demand. This is a relatively inefficient operation, however. It normally involves a storage controller of the storage system first copying the data from the first tier into the cache, and then writing the data again to the second tier. This frees up space in the first tier where the data was previously stored, but comes at the cost of many additional storage device (e.g., read and write) operations. This imposes a burden on the number of operations the controller performs, as well as potentially prematurely exhausts the life span of media that are designed to endure a limited number of writes.

DETAILED DESCRIPTION

All examples and illustrative references are non-limiting and should not be used to limit the claims to specific implementations and embodiments described herein and their equivalents. For simplicity, reference numbers may be repeated between various examples. This repetition is for clarity only and does not dictate a relationship between the respective embodiments. Finally, in view of this disclosure, particular features described in relation to one aspect or embodiment may be applied to other disclosed aspects or embodiments of the disclosure, even though not specifically shown in the drawings or described in the text.

Various embodiments include systems, methods, and machine-readable media for improving the operation of storage systems by reducing the number of operations that are performed for data migration between different redundancy levels. In an exemplary embodiment, a multi-tier system includes a first tier having a volume with a redundancy type suitable for high performance and a second tier having a volume with a redundancy type suitable for capacity efficiency. For example, the first tier volume type may be a RAID 10 and the second tier volume type may be a RAID 5 or RAID 6. The first tier volume and the second tier volume may be built from the same set of storage devices in a storage pool, such that each storage device involved includes both the first and second tier volumes.

When data is sent to the storage system for storage, referred to herein as write data, the storage system stores the write data initially to the first tier volume to facilitate high performance access to the write data. When demand for the write data declines, for example by falling below a threshold, the storage system may migrate the write data to the second tier volume for capacity efficiency. To perform the migration, the storage system may directly swap data extents from the first tier volume to the second tier volume with unused data extents from the second tier volume to the first tier volume. In other words, the storage system changes the mapping of underlying physical locations on the storage devices where the write data is stored so that the underlying physical locations are logically associated with the second tier volume instead of the first tier volume. After remapping, the storage system also computes parity information for the migrated write data and stores it in the second tier volume.

As a result of this swapping/remapping, the storage system's performance is improved by reducing the number of operations necessary to migrate the write data. Further, the overall number of blocks written to storage devices is reduced, reducing wear on the storage devices themselves which may have limited endurance.

FIG. 1illustrates a data storage architecture100in which various embodiments may be implemented. The storage architecture100includes a storage system102in communication with a number of hosts104. The storage system102is a system that processes data transactions on behalf of other computing systems including one or more hosts, exemplified by the hosts104. The storage system102may receive data transactions (e.g., requests to write and/or read data) from one or more of the hosts104, and take an action such as reading, writing, or otherwise accessing the requested data. For many exemplary transactions, the storage system102returns a response such as requested data and/or a status indictor to the requesting host104. It is understood that for clarity and ease of explanation, only a single storage system102is illustrated, although any number of hosts104may be in communication with any number of storage systems102.

While the storage system102and each of the hosts104are referred to as singular entities, a storage system102or host104may include any number of computing devices and may range from a single computing system to a system cluster of any size. Accordingly, each storage system102and host104includes at least one computing system, which in turn includes a processor such as a microcontroller or a central processing unit (CPU) operable to perform various computing instructions. The instructions may, when executed by the processor, cause the processor to perform various operations described herein with the storage controllers108.a,108.bin the storage system102in connection with embodiments of the present disclosure. Instructions may also be referred to as code. The terms “instructions” and “code” may include any type of computer-readable statement(s). For example, the terms “instructions” and “code” may refer to one or more programs, routines, sub-routines, functions, procedures, etc. “Instructions” and “code” may include a single computer-readable statement or many computer-readable statements.

The processor may be, for example, a microprocessor, a microprocessor core, a microcontroller, an application-specific integrated circuit (ASIC), etc. The computing system may also include a memory device such as random access memory (RAM); a non-transitory computer-readable storage medium such as a magnetic hard disk drive (HDD), a solid-state drive (SSD), or an optical memory (e.g., CD-ROM, DVD, BD); a video controller such as a graphics processing unit (GPU); a network interface such as an Ethernet interface, a wireless interface (e.g., IEEE 802.11 or other suitable standard), or any other suitable wired or wireless communication interface; and/or a user I/O interface coupled to one or more user I/O devices such as a keyboard, mouse, pointing device, or touchscreen.

With respect to the storage system102, the exemplary storage system102contains any number of storage devices106and responds to one or more hosts104's data transactions so that the storage devices106may appear to be directly connected (local) to the hosts104. In various examples, the storage devices106include hard disk drives (HDDs), solid state drives (SSDs), optical drives, and/or any other suitable volatile or non-volatile data storage medium. In some embodiments, the storage devices106are relatively homogeneous (e.g., having the same manufacturer, model, and/or configuration). However, the storage system102may alternatively include a heterogeneous set of storage devices106that includes storage devices of different media types from different manufacturers with notably different performance.

The storage system102may group the storage devices106for speed and/or redundancy using a virtualization technique such as RAID or disk pooling (that may utilize a RAID level). The storage system102also includes one or more storage controllers108.a,108.bin communication with the storage devices106and any respective caches. The storage controllers108.a,108.bexercise low-level control over the storage devices106in order to execute (perform) data transactions on behalf of one or more of the hosts104. The storage controllers108.a,108.bare illustrative only; more or fewer may be used in various embodiments. Having at least two storage controllers108.a,108.bmay be useful, for example, for failover purposes in the event of equipment failure of either one. The storage system102may also be communicatively coupled to a user display for displaying diagnostic information, application output, and/or other suitable data.

In an embodiment, the storage system102may group the storage devices106using a dynamic disk pool (DDP) (or other declustered parity) virtualization technique. In a dynamic disk pool, volume data, protection information, and spare capacity are distributed across all of the storage devices included in the pool. As a result, all of the storage devices in the dynamic disk pool remain active, and spare capacity on any given storage device is available to all volumes existing in the dynamic disk pool. Each storage device in the disk pool is logically divided up into one or more data extents at various logical block addresses (LBAs) of the storage device. A data extent is assigned to a particular data stripe of a volume. An assigned data extent becomes a “data piece,” and each data stripe has a plurality of data pieces, for example sufficient for a desired amount of storage capacity for the volume and a desired amount of redundancy, e.g. RAID 0, RAID 1, RAID 10, RAID 5 or RAID 6 (to name some examples). As a result, each data stripe appears as a mini RAID volume, and each logical volume in the disk pool is typically composed of multiple data stripes.

In the present example, storage controllers108.aand108.bare arranged as an HA pair. Thus, when storage controller108.aperforms a write operation for a host104, storage controller108.amay also send a mirroring I/O operation to storage controller108.b. Similarly, when storage controller108.bperforms a write operation, it may also send a mirroring I/O request to storage controller108.a. Each of the storage controllers108.aand108.bhas at least one processor executing logic to perform writing and migration techniques according to embodiments of the present disclosure.

Moreover, the storage system102is communicatively coupled to server114. The server114includes at least one computing system, which in turn includes a processor, for example as discussed above. The computing system may also include a memory device such as one or more of those discussed above, a video controller, a network interface, and/or a user I/O interface coupled to one or more user I/O devices. The server114may include a general purpose computer or a special purpose computer and may be embodied, for instance, as a commodity server running a storage operating system. While the server114is referred to as a singular entity, the server114may include any number of computing devices and may range from a single computing system to a system cluster of any size. In an embodiment, the server114may also provide data transactions to the storage system102. Further, the server114may be used to configure various aspects of the storage system102, for example under the direction and input of a user. Some configuration aspects may include definition of RAID group(s), disk pool(s), and volume(s), to name just a few examples.

With respect to the hosts104, a host104includes any computing resource that is operable to exchange data with a storage system102by providing (initiating) data transactions to the storage system102. In an exemplary embodiment, a host104includes a host bus adapter (HBA)110in communication with a storage controller108.a,108.bof the storage system102. The HBA110provides an interface for communicating with the storage controller108.a,108.b, and in that regard, may conform to any suitable hardware and/or software protocol. In various embodiments, the HBAs110include Serial Attached SCSI (SAS), iSCSI, InfiniBand, Fibre Channel, and/or Fibre Channel over Ethernet (FCoE) bus adapters. Other suitable protocols include SATA, eSATA, PATA, USB, and FireWire.

The HBAs110of the hosts104may be coupled to the storage system102by a network112, for example a direct connection (e.g., a single wire or other point-to-point connection), a networked connection, or any combination thereof. Examples of suitable network architectures112include a Local Area Network (LAN), an Ethernet subnet, a PCI or PCIe subnet, a switched PCIe subnet, a Wide Area Network (WAN), a Metropolitan Area Network (MAN), the Internet, Fibre Channel, or the like. In many embodiments, a host104may have multiple communicative links with a single storage system102for redundancy. The multiple links may be provided by a single HBA110or multiple HBAs110within the hosts104. In some embodiments, the multiple links operate in parallel to increase bandwidth.

To interact with (e.g., write, read, modify, etc.) remote data, a host HBA110sends one or more data transactions to the storage system102. Data transactions are requests to write, read, or otherwise access data stored within a data storage device such as the storage system102, and may contain fields that encode a command, data (e.g., information read or written by an application), metadata (e.g., information used by a storage system to store, retrieve, or otherwise manipulate the data such as a physical address, a logical address, a current location, data attributes, etc.), and/or any other relevant information. The storage system102executes the data transactions on behalf of the hosts104by writing, reading, or otherwise accessing data on the relevant storage devices106. A storage system102may also execute data transactions based on applications running on the storage system102using the storage devices106. For some data transactions, the storage system102formulates a response that may include requested data, status indicators, error messages, and/or other suitable data and provides the response to the provider of the transaction.

Data transactions are often categorized as either block-level or file-level. Block-level protocols designate data locations using an address within the aggregate of storage devices106. Suitable addresses include physical addresses, which specify an exact location on a storage device, and virtual addresses, which remap the physical addresses so that a program can access an address space without concern for how it is distributed among underlying storage devices106of the aggregate. Exemplary block-level protocols include iSCSI, Fibre Channel, and Fibre Channel over Ethernet (FCoE). iSCSI is particularly well suited for embodiments where data transactions are received over a network that includes the Internet, a WAN, and/or a LAN. Fibre Channel and FCoE are well suited for embodiments where hosts104are coupled to the storage system102via a direct connection or via Fibre Channel switches. A Storage Attached Network (SAN) device is a type of storage system102that responds to block-level transactions.

In contrast to block-level protocols, file-level protocols specify data locations by a file name. A file name is an identifier within a file system that can be used to uniquely identify corresponding memory addresses. File-level protocols rely on the storage system102to translate the file name into respective memory addresses. Exemplary file-level protocols include SMB/CFIS, SAMBA, and NFS. A Network Attached Storage (NAS) device is a type of storage system that responds to file-level transactions. It is understood that the scope of present disclosure is not limited to either block-level or file-level protocols, and in many embodiments, the storage system102is responsive to a number of different memory transaction protocols.

According to embodiments of the present disclosure, the storage system102may include multiple storage tiers, with each tier having a different redundancy level. This is illustrated, in one example, inFIG. 2, which is an organizational diagram of an exemplary controller architecture for a storage system102according to aspects of the present disclosure. Additionally, and as explained in more detail below, various embodiments include the storage controllers108.aand108.bexecuting computer readable code to perform the stripe swapping operations described herein.

FIG. 2illustrates a first storage tier202and a second storage tier204. Embodiments of the present disclosure may include more than two tiers as well without departing from the scope of the present disclosure. The first storage tier202and the second storage tier204may each be in communication with both of the redundant storage controllers108.a,108.b. The first storage tier202may be implemented with a RAID level (or other type of organization that may implement features similar to RAID levels) that provides relatively better performance as compared with other RAID levels. For example, the first storage tier202may be implemented with a RAID 0, RAID 1, or RAID 10 level. For purposes of discussion here, the first storage tier202will be described as being implemented as a dynamic disk pool implementing a redundancy similar to RAID 10 (e.g., a fast RAID level that includes redundancy). The first storage tier202may include one or more volumes, for example volumes that logically may span one or more physical storage devices106.

The second storage tier204may be implemented with a RAID level (or other type of organization that may implement features similar to RAID levels) that provides relatively better capacity utilization (usually at the expense of speed performance) as compared with the RAID level type implemented for the first storage tier202. For example, the second storage tier204may be implemented with a RAID 5 or RAID 6 level. For purposes of discussion here, the second storage tier204will be described as being implemented as a DDP implementing a redundancy similar to RAID 5. The second storage tier204may also include one or more volumes, for example logical volumes that logically may span one or more physical storage devices106.

An exemplary physical storage device106with multiple data extents is illustrated inFIG. 2as well. According to embodiments of the present disclosure, a given storage device106may be logically partitioned into multiple volumes. Thus, inFIG. 2a first portion of the physical storage device106may be associated with a first volume of the first storage tier202and a second portion with a second volume of the second storage tier204. A single physical storage device106is illustrated for ease of demonstration inFIG. 2. A typical implementation may incorporate any number of these physical storage devices106, for example according to DDP/RAID techniques a different data extent of a given data stripe in a storage tier may be stored in each of multiple physical storage devices106. Also, the following examples inFIGS. 2-4Dare given with respect to one storage controller (108.aor108.b) performing the actions, where the example refers to storage controller108. However, it is understood that the other one of the storage controllers108.aor108.bhas the same capabilities and may perform the same operations when it executes a read or write operation. For example, each storage controller may maintain its own mapping tables and may perform its own read and write operations received from various ones of the hosts.

According to embodiments of the present disclosure, when a host104sends write data to the storage system102, a storage controller108receives the write request/data and writes the data to a data stripe of the first storage tier202, so that better write performance may be obtained. The write data may then be maintained at the first storage tier202for a period of time that the write data is frequently accessed, e.g. has an access frequency above a first threshold (such as a number of accesses during a fixed period of time). When the frequency of access falls, and/or after a specified period of time expires, the storage controller108may migrate the write data from the first storage tier202to the second storage tier204, so that the data may be stored for a longer term in a volume more suitable to long term storage.

For the migration, the storage controller108looks to what data extents of the second storage tier204are available from the same physical storage devices106where the data extents of the first storage tier202already are associated with the write data. For example, the write data may be stored on first, second, third, and fourth data extents associated with first, second, third, and fourth physical storage devices106for the first storage tier202. As a result, the storage controller108may look to determine which data extents associated with (also referred to as being mapped to) the second storage tier204on the same physical storage devices106are available for swapping the data stripe containing the write data.

Once the storage controller108has identified available data extents mapped to the same physical storage devices106for the second storage tier204, the storage controller108may update the mapping information in mapping tables maintained for each of the first storage tier202and the second storage tier204. For the update, the storage controller108associates the data extents currently mapped to the locations on the corresponding physical storage devices106storing the write data to the second storage tier204(to locations on the same physical storage devices106that are “empty”—whether truly empty or available to have their existing data overwritten). The storage controller108further associates the data extents currently mapped to the “empty” locations to the first storage tier202, specifically to the locations on the corresponding physical storage devices106storing the write data.

As a result of the mapping change for the data stripe containing the write data, the storage controller108of the storage system102may migrate the write data to the second storage tier204from the first storage tier202without having to further move the write data itself. The above-noted aspects of the present disclosure do not require the storage controller108to read out the write data from the first storage tier202into cache and re-write it to the second storage tier204. Rather, the write data remains in the same physical locations on the physical storage devices106, while the logical mappings change to reflect the desired storage tier.

FIG. 3is a diagram illustrating an exemplary mapping table300according to aspects of the present disclosure. As noted above, the mapping table300may be a table maintained by a storage controller108of the storage system102. It may be kept in a cache of the storage controller108and/or longer-term storage. There may be a single mapping table that the storage controller108maintains for multiple storage tiers. Alternatively, each storage tier may have its own mapping table that is maintained by the storage controller108. For example, the storage controller108may maintain a first mapping table300for tracking the first storage tier202ofFIG. 2and a second mapping table300for tracking the second storage tier204.

As illustrated, the mapping table300is a two-dimensional table where each row corresponds to a different data stripe (or multiple data stripes)308. A data stripe308may have a plurality of data pieces, for example sufficient for a desired amount of storage capacity for the volume and a desired amount of redundancy. As illustrated, the rows go in order of stripe number302, e.g. in increasing order of value. As further illustrated, the columns correspond to different data extents306and go in order of extent number304. Each storage device may be logically divided up into one or more data extents at various logical block addresses (LBAs) of the storage device. For example, that may be in order of increasing value (e.g., 1, 2, 3, . . . ). With this combination (stripe number and extent number), the storage controller108is able to locate information at a specified LBA found in a given stripe/data extent combination.

When the storage controller108is performing a data migration according to embodiments of the present disclosure, the storage controller108may access the mapping table300for the first storage tier202, locate the entries in the table corresponding to the write data to be migrated, and remove those entries from the table. The storage controller108may at approximately the same time (or subsequently) access the mapping table300for the second storage tier204and locate entries in the table corresponding to available data extents of the second storage tier204and remove those entries from the second table.

The storage controller108places the entries corresponding to the write data into the mapping table300for the second storage tier204in place of the entries that used to correspond to available data extents. Similarly, the storage controller108places the entries corresponding to the available data extents into the mapping table for the first storage tier202in place of the entries that previously corresponding to the write data that has now been migrated. As a result, the pointers in the tables corresponding to the first and second storage tiers202,204now point to different physical locations for the migrated data stripes, while the write data itself did not have to be re-written for the migration to occur.

An example of how embodiments of the present disclosure operate to migrate data between storage tiers while reducing the number of disk operations necessary to migrate the data (and, thereby, increase system performance) is now discussed with respect to the storage device arrangement400illustrated inFIGS. 4A, 4B, 4C, and 4D. The example storage device arrangement400illustrated in these figures is for ease of discussion only. In these figures, eight storage devices106a,106b,106c,106d,106e,106f,106g, and106hare illustrated. More or fewer may be included as will be recognized. As illustrated here, the storage tiers202,204correspond to respective volumes (in other words, the volumes are co-extant with the storage tiers they are associated with, such that reference to the storage tier202is also reference to the volume having the first redundancy level and reference to the storage tier204is also reference to the volume having the second redundancy level) in this example.

In the example illustrated inFIGS. 4A, 4B, 4C, and 4D, the first storage tier202has implemented a DDP RAID 10 architecture, but is arranged with the data extents grouped together (across storage devices106for a given stripe) followed by corresponding mirror drive extents. Other RAID types could alternatively be implemented that are faster than the RAID types that focus more on capacity utilization. The second storage tier204has implemented a DDP RAID 5 architecture, here a 4+1 (four data with one parity extent) architecture. In an embodiment, this may involve a de-cluster parity.

As noted above, the first storage tier202is implemented on the same physical storage devices106as the second storage tier204so that data migration may occur without having to rewrite the data between tiers.FIG. 4Aillustrates this with the second storage tier204occupying a first range of LBAs on the storage devices106a,106b,106c,106d,106e,106f,106g, and106h, the first storage tier202occupying a second range of LBAs on the storage devices106a,106b,106c,106d,106e,106f,106g, and106h, and a gap range of LBAs404between them as well as potentially after the first storage tier202on any given storage device (and/or potentially before the second storage tier204). Although the second storage tier204is illustrated as occupying a lower LBA range than the first storage tier202, the placement may be reversed such as the first storage tier occupies a lower LBA range than the second storage tier204.

InFIG. 4A, data is not stored yet in the physical locations associated with the data extents A1through A44of the second storage tier204. Write data402is received and goes to the first storage tier202(in this embodiment, the write data first goes towards the first storage tier202before later migration to the second, usually slower second storage tier204). When the storage controller108receives the write data402, it initially writes it to data extents B1, B2, B3and B4in the first storage tier202(illustrated with the box fill inFIG. 4A). Additionally, according to the RAID redundancy level of the first storage tier202in this example, the storage controller108also creates a mirror of the write data and writes it to data extents B1′, B2′, B3′, and B4′.

In an embodiment, such as the one illustrated inFIGS. 4A, 4B, 4C, and 4D, the data extents of the write data and mirror data in the first storage tier202are contiguous to each other—or, in other words, data extents are not left unused between the write data at B1, B2, B3, and B4and the mirror data B1′, B2′, B3′, and B4′. As a result, the capacity of the first storage tier202may be better utilized. However, this leaves a mismatch between the first and second storage tiers202,204for subsequent data stripes. For example, since the second storage tier204is implementing a 4+1 volume, the storage controller108may have to search multiple ranges of data extents in order to identify corresponding available data extents between the first and second storage tiers.

For example, in theFIG. 4Aembodiment, data extents B1, B2, B3, and B4are on the same storage devices106a,106b,106c, and106das data extents A1, A2, A3, and A4of the second storage tier204. Thus, these are available candidates for stripe swapping during data migration. With the additional parity extent on storage device106e, there arises a mismatch between the next stripe of write data stored to the first storage tier202(the mismatch being the availability of the same number of data extents of the second storage tier204on the same storage devices106as the data extents of the first storage tier202that may be migrating). Therefore, the storage controller108may skip certain sections of data extents of the second storage tier204until a suitable set of data extents are unused and available to be swapped as a stripe to the first storage tier202(which may be the data extents A33, A34, A35, and A36in the example ofFIG. 4A, leaving the intervening data extents potentially unused or filled with other data not yet migrated from the first storage tier202).

In an alternative embodiment, one or more data extents of the first storage tier202may be left unused (e.g., a gap of one or more data extents between the write data B1-B4and the mirror data B1′-B4′). This facilitates a direct mapping consistently between the different storage volumes on the same storage devices106. This removes the potential for mismatch between the stripes of write data in the first storage tier202and the availability of unused data extents (stripes) in the second storage tier204. This comes at the cost of potentially unused data extents in the first storage tier202, which may already suffer from a reduced capacity capability depending upon the RAID type implemented.

Turning now toFIG. 4B, after some time (whether a short period or longer), the storage controller108receives write data406and initially writes it to data extents B5, B6, B7and B8in the first storage tier202. Additionally, according to the RAID redundancy level of the first storage tier202in this example, the storage controller108also creates a mirror of the write data and writes it to data extents B5′, B6′, B7′, and B8′. InFIG. 4B, according to the example, the write data402remains in the first storage tier202because it is frequently accessed, for example above a predetermined number of times (whether read or modify) over a predetermined period of time. Thus, no data migration between tiers occurs yet.

Turning now toFIG. 4C, after some time from the events ofFIG. 4B, the storage controller108receives write data408and initially writes it to data extents B9, B10, B11, and B12in the first storage tier202. Additionally, according to the RAID redundancy level of the first storage tier202in this example, the storage controller108also creates a mirror of the write data and writes it to data extents B9′, B10′, B11′, and B12′. In conjunction with receipt of the write data408, or separate from, the storage controller108may determine that the write data402stored at data extents B1, B2, B3, and B4is no longer “hot,” or accessed a number of times that exceeds the predetermined threshold.

As a result, the storage controller108may determine that the write data402should be migrated to the second storage tier204. To that end, the storage controller108identifies data extents in the second storage tier204that are both unused and correspond to the same storage devices106where the data extents of the first storage tier202are located (here, of storage devices106a,106b,106c, and106dthat correspond to where the data extents B1, B2, B3, and B4are currently mapped). In the example ofFIG. 4C, the storage controller108may identify the data extents A1, A2, A3, and A4of the second storage tier204as unused and therefore available for the data stripe migration. Since a 4+1 RAID 5 type is being implemented in this example, the storage controller108may also confirm that a data extent is also available (e.g., unused) for storing parity information.

Once the storage controller108has identified, or located, data extents for the migration, the storage controller108may proceed with changing the mapping information for the data stripes. For example, the storage controller108may access a mapping table for the first storage tier202, unmap the data extents/data stripe corresponding to the write data in the first storage tier202's mapping table (e.g., the table300ofFIG. 3), and remap the data extents/data stripe corresponding to the unused device106ranges from the second storage tier204's mapping table to the first storage tier202's mapping table. Similarly, the storage controller108may access a mapping table for the second storage tier204, unmap the data extents/data stripe corresponding to the unused portions (e.g., in a corresponding table300), and remap the data extents/data stripe corresponding to the write data to the second storage tier204's mapping table. This remapping is illustrated inFIG. 4Cwith the bi-directional arrows for these data extents, indicating that they are swapped with each other in the first and second storage tiers202,204without changing the data actually stored at the underlying physical locations.

After the mapping information in the respective mapping tables are changed/updated, the storage controller108may compute parity information410for the data extents B1, B2, B3, and B4that are now mapped to the second storage tier204. The parity information410is stored adjacent to the data extent A4as parity extent P(1-4) associated with storage device106e. Further, since the data extents B1, B2, B3, and B4corresponding to the write data have been migrated as a data stripe to the second storage tier204, the mirror data remaining in the first storage tier202becomes unnecessary (for example, after the parity information has been computed and stored). The data extents B1′, B2′, B3′, and B4′ may therefore be reclaimed (e.g., by the storage controller108) and reused (together with the unused A1, A2, A3, and A4data extents now mapped to the first storage tier202) for future writes to the first storage tier202.

Because the storage devices106have multiple volumes associated with the multiple storage tiers on each of them, and data is remapped to data extents of the other storage tier(s) on the same physical devices106, the number of writes may be noticeably reduced. In the example ofFIG. 4C, there are 4 reads (corresponding to the 4 data extents with write data) and one write (the parity information410stored after the remapping). In contrast, movement of the underlying data to a different location corresponding to the second storage tier204would have resulted in 4 reads and 5 writes (the four data extents with the write data plus the parity information).

The results of this migration may be seen inFIG. 4D. As illustrated inFIG. 4D, the write data migrated to the second storage tier204is now maintained at data extents A1, A2, A3, and A4(with parity information at P(1-4)). Thus, these logical data extents are now occupied in the second storage tier204. Further, data extents B1, B2, B3, B4, B1′, B2′, B3′, and B4′ are now unused in the first storage tier202and may be written to (for example, the storage controller108may cyclically work through the LBA ranges of the first storage tier202and eventually loop back to the now unused data extents). The storage controller108may continue initially storing incoming write data in the first storage tier202and then migrate the data according to embodiments of the present disclosure after some period of time has passed and/or access frequency threshold passed.

FIG. 5is a flow diagram of a method500for performing data migration through stripe swapping according to aspects of the present disclosure. In an embodiment, the method500may be implemented by one or more processors of one or more of the storage controllers108of the storage system102, executing computer-readable instructions to perform the functions described herein. In the description ofFIG. 5, reference is made to a storage controller108(108.aor108.b) for simplicity of illustration, and it is understood that other storage controller(s) may be configured to perform the same functions when performing a read or write operation. It is understood that additional steps can be provided before, during, and after the steps of method500, and that some of the steps described can be replaced or eliminated for other embodiments of the method500.

At block502, the storage controller108receives write data from a host104. The storage controller108stores the write data to a first storage tier, such as first storage tier202discussed above with respect toFIGS. 2, 3, 4A, 4B, 4C, and 4D. For example, the write data may be written to physical storage locations mapped to data extents B1, B2, B3, and B4in the example ofFIGS. 4A, 4B, 4C, and 4D. Further, the storage controller108may update entries in a mapping table maintained in correspondence with the first storage tier, for example a mapping table300.

At block504, the storage controller108creates a mirror of the received write data and stores it in the first storage tier following the storage of the write data.

At block506, the storage controller108tracks an access frequency to the write data as the write data is stored in the first storage tier. For example, the storage controller108may track a number of read and/or modify requests for the write data over a predetermined period of time. For instance, the storage controller108may store and maintain metadata that indicates a number of read and/or modify requests to memory. The storage controller108may then analyze that metadata to determine a number of read and/or modify requests.

At block508, the storage controller108compares the tracked access frequency for the write data to a predetermined first threshold. For example, the storage controller108may store in a cache or other memory the predetermined first threshold and access the stored information for the comparison. The threshold may be set during volume configuration or at some other appropriate time.

At decision block510, the storage controller108determines whether the comparison at block508resulted in the tracked access frequency being less than (or, in embodiments, less than or equal to) the predetermined first threshold. If not, then the method500may return to block506to continue monitoring as discussed above.

If it is instead determined at decision block510that the tracked access frequency is less than (or less than or equal to) the predetermined first threshold, then the method500proceeds to block512.

At block512, the storage controller108locates an unused data stripe in a second storage tier (for example, a storage tier having a redundancy level different from the redundancy level of the first storage tier, such as a parity redundancy type like RAID 5 or RAID 6), where the data stripe has data extents associated with the same storage devices as the data stripe in the first storage tier that is storing the write data. Where the first storage tier has a mirror type of redundancy and the second storage tier has a parity type of redundancy, the storage controller108may also check to confirm that there is an additional data extent available in the data stripe in the second storage tier.

At block514, the storage controller108remaps the data stripe containing the write data to the second storage tier instead of the first storage tier, as well as remaps the unused data stripe from the second storage tier to the first storage tier. The remapping occurs by the storage controller108changing the mapping in the mapping tables maintained for the respective first and second storage tiers, so that the data stripe containing the write data is now mapped with the second storage tier and the data stripe that was unused is now mapped with the first storage tier. Further, the storage controller108may read the data associated with each of the data extents being remapped to the second storage tier and compute parity information for it. Once the parity information is computed, the storage controller108stores the parity information in the data extent identified at block512. As a result, the write data itself remains stored in the same physical locations on the storage devices and re-associated logically with the second storage tier. This reduces the number of writes to just the parity information.

At block516, the storage controller108releases the mirrored data corresponding to the write data previously associated with the first storage tier. As a result of this release, the data stripe previously occupied by this data may be reclaimed and reused for future writes to the first storage tier.

At block518, the storage controller108tracks an access frequency to the write data as the write data is now stored (logically) in the second storage tier. For example, the storage controller108may track a number of read and/or modify requests for the write data over a predetermined period of time. For instance, the storage controller108may store and maintain metadata that indicates a number of read and/or modify requests to memory. The storage controller108may then analyze that metadata to determine a number of read and/or modify requests.

At block520, the storage controller108compares the tracked access frequency for the write data to a predetermined second threshold. For example, the storage controller108may store in a cache or other memory the predetermined second threshold and access the stored information for the comparison. In an embodiment, the predetermined second threshold may be a value greater than the predetermined first threshold so that an element of hysteresis is built into the system. Alternatively, the second threshold may be equal to the first threshold. The threshold may be set during volume configuration or at some other appropriate time.

At decision block522, the storage controller108determines whether the comparison at block520resulted in the tracked access frequency being greater than (or, in embodiments, greater than or equal to) the predetermined second threshold. If not, then the method500may return to block518to continue monitoring as discussed above.

If it is instead determined at decision block522that the tracked access frequency is greater than (or greater than or equal to) the predetermined second threshold, then the method500proceeds to block524. This corresponds to situations where the write data becomes more in demand, indicating that it may be worthwhile to migrate the write data back to the first storage tier for potentially improved access speed.

At block524, the storage controller108locates an unused data stripe in the first storage tier, where the data stripe has data extents associated with the same storage devices as the data stripe in the second storage tier that is currently storing the write data. In an embodiment, this may also include checking to confirm that there are additional data extents, corresponding in number to the data extents where the write data is stored, available as well. For example, where the write data occupies 4 data extents, the storage controller108may check whether 8 data extents, 4 for the write data and 4 for the mirror of the write data, are unused.

At block526, once an unused data stripe is located, the storage controller108remaps the data stripe containing the write data to the first storage tier instead of the second storage tier, as well as remaps the unused data stripe from the first storage tier to the second storage tier (basically, the reverse operation of the remapping discussed with respect to block514above). The remapping occurs in the mapping tables maintained for the respective first and second storage tiers.

Further, at block528the storage controller108reads the data associated with each of the data extents remapped to the first storage tier and a generates a mirrored copy for the first storage tier. The mirrored write data is stored in unused data extents in the first storage tier identified from block524.

As a result of the elements discussed above, a storage system's performance is improved by reducing the number of operations necessary to migrate write data from a high performance storage tier to a capacity efficient storage tier. Further, the overall number of blocks written to storage devices is reduced, reducing wear on the storage devices themselves which may have limited endurance.

The present embodiments can take the form of a hardware embodiment, a software embodiment, or an embodiment containing both hardware and software elements. In that regard, in some embodiments, the computing system is programmable and is programmed to execute processes including the processes of method500discussed herein. Accordingly, it is understood that any operation of the computing system according to the aspects of the present disclosure may be implemented by the computing system using corresponding instructions stored on or in a non-transitory computer readable medium accessible by the processing system. For the purposes of this description, a tangible computer-usable or computer-readable medium can be any apparatus that can store the program for use by or in connection with the instruction execution system, apparatus, or device. The medium may include for example non-volatile memory including magnetic storage, solid-state storage, optical storage, cache memory, and Random Access Memory (RAM).