Patent Description:
Computing systems typically require that data be stored and retrieved. A storage location or medium that a computing system may store data to is often termed a "drive". A traditional example of a drive is a physical hard drive. However, with the advent of networks and then cloud computing, various technologies have developed to present the appearance of a hard drive (a virtual hard drive) to a computing system, even though the actual storage used to emulate the hard drive is abstracted away from view, and may even be quite distributed.

Before a computing system may use a drive, the drive must be mounted to the computing system. Mounting is a process by with the structure on the drive is made accessible via the storage channels (e.g., a local file system) available on the computing system. Such a point of access is referred to as a "mount point". When mounting a writable volume, mounting also ensures that the computing system is capable of writing data to the drive honoring existing data structures within the drive thereby avoiding corruption of the drive. When mounting a readable volume, mounting ensures that the computing system may properly interpret the structure of the drive.

<CIT> relates to managing virtual hard drives in a cloud computing platform. The cloud computing platform that includes fabric computers and blob stores that are used to access the virtual hard drives. A CreateDrive command is used by the cloud application to create the virtual hard drive. The cloud application specifies the size and format for the virtual hard drive. A MountDrive command is used by the cloud application to mount the VHD. The cloud application may also request an exclusive write VHD, write VHD, shared read only VHD etc..

It is the object of the present invention to provide a method and system for mounting a drive with reduced risk of corruption.

At least some embodiments described herein relate to the mounting a drive to two or more computing systems. For instance, the drive may be mounted to a first computing system so as to be writable (and potentially readable) by the first computing system. But also, the drive is mounted to one or more other computing systems so as to be only readable by those one or more computing systems. This allows for multiple computing systems to have access to the drive without risk that the data on the drive will become corrupted. In one embodiment, the only user data stored on that drive is a single file of fixed size. Thus, even when user data is written into the fixed-size file, the management data stored (that keeps track of the files) on the drive does not change.

This mounting of a single drive to multiple computing systems may be especially beneficial in the case of there being a primary computing system that is supported by one or more fallback secondary computing systems. This might be the case when the primary computing system is running a very important process for which backup is necessary by one or more secondary computing systems. It is common in a variety of cloud applications to have a primary computing system with multiple secondary computing systems. If the primary computing system fails, failover occurs to one of the secondary computing systems, which will continue the process.

This ensures that the process has sufficient redundancy so that the process is highly available. In this case, the primary computing system is operating upon the data, while the secondary computing systems read data preparing for the day they might be called upon to become the primary computing system. If failover is necessary, one of the secondary computing systems may be selected as the primary, and the mounting of the drive to the secondary computing system may be augmented to a writable mounting also, or the secondary computing system may simply be notified that it is now the primary computing system. This significantly reduces the time required to perform failover since a drive need not now be mounted to the secondary computing system as part of the failover. The drive is already present, and available.

Therefore, these drawings depict only example embodiments of the invention and are not therefore to be considered to be limiting of the scope of the invention. With this in mind, example embodiments of the invention will be described and explained with reference to the accompanying drawings in which:.

At least some embodiments described herein relate to the mounting a drive to two or more computing systems. For instance, the drive may be mounted to a first computing system so as to be writable (and potentially readable) by the first computing system. But also, the drive is also mounted to one or more other computing systems so as to be only readable by those one or more computing systems. This allows for multiple computing systems to have access to the drive without risk that the data thereon will become corrupted. In one embodiment, the only user data stored on that drive is a single file of fixed size. Thus, even when user data is written into the fixed-size file, the management data (that keeps track of the files) stored on the drive does not change.

<FIG> illustrates an environment <NUM> that includes a mounting computing system <NUM> that mounts a drive <NUM> to multiple computing systems <NUM>. The mounting computing system <NUM> may be a computing system such as the computing system <NUM> described below with respect to <FIG>. Each of the multiple computing systems <NUM> may also be a computing system such as the computing system <NUM> described below with respect to <FIG>. The mounting computing system <NUM> may be a separate computing system, or may be one of the computing systems <NUM> (such as computing system <NUM>). Alternatively, the mounting computing system <NUM> may be a larger computing environment that actually includes the computing system <NUM>, the computing system <NUM>, the computing system <NUM>, and/or the drive <NUM>.

The drive <NUM> may be a physical hard drive. Alternatively, the drive <NUM> may be a virtual hard drive. A virtual hard drive is a component that offers the appearance and behavior of (i.e., emulates) a hard drive to a computing system mounted thereto, even though the actual storage used to emulate the hard drive is abstracted away from view of the computing system, and may even be quite distributed.

The computing systems <NUM> and <NUM> may each be physical computing systems. Alternatively, one, some, or all of the computing systems <NUM> and <NUM> may each be a virtual machine. A virtual machine is a component that offers the appearance and behavior of (i.e., emulates) a computing system to a user, even though the functions of that virtual machine are performed by physical components that are abstracted away from the view of the user. Ideally, a user might not be able to tell that the user is working on a virtual machine as opposed to a physical computing system that performs the same function as the virtual machine.

The bi-direction dashed arrow <NUM> represents that the drive <NUM> is mounted as a writable (and potentially readable) drive for the computing system <NUM>. The one-directional dotted arrow <NUM> represents that the drive <NUM> is mounted as only a readable drive for the computing system <NUM>. Likewise, the one-directional dotted arrow <NUM> represents that the drive <NUM> is mounted as only a readable drive for the computing system <NUM>. The ellipsis <NUM> represents that there is flexibility in the number of the computing systems <NUM> to which the drive <NUM> is mounted. The drive <NUM> may of course be mounted to just a single computing system. However, in accordance with the principles described herein, the drive may also be mounted to any plurality number of computing systems. In this case, there is one computing system that may write to the drive, whereas the other computing system(s) may only read from the drive. This avoids write corruption that would normally be expected when mounting a drive to multiple computing systems.

<FIG> illustrates a flowchart of a method <NUM> for mounting a drive in accordance with the principles described herein. As an example, the method <NUM> may be performed by the mounting computing system <NUM> in order to mount the drive <NUM> to multiple computing systems, such as the computing systems <NUM>. Thus, the method <NUM> will be described with reference to both <FIG> and <FIG>. The method <NUM> may be performed by, for instance, one or more processors of the mounting computing system <NUM> executing one or more instructions that are structured such that, when executed by the one or more processors, the mounting computing system <NUM> is caused to perform the method <NUM>.

The method <NUM> includes mounting the drive to a first computing system so as to be writable by the first computing system (act <NUM>). For instance, in <FIG>, the drive <NUM> is mounted to the computing system <NUM> as represented by the arrow <NUM>. The computing system <NUM> may thus write to (and potentially also read from) the drive <NUM>. The method <NUM> also includes mounting the drive <NUM> to a second computing system so as to be readable by the second computing system (act <NUM>). For instance, in <FIG>, the drive <NUM> is mounted to the computing system <NUM> as represented by the arrow <NUM>. Thus, the first computing system and the second computing system both have concurrent access to the drive. Of course, to avoid the rare circumstance in which the computing system <NUM> attempts to read from data that is in the midst of being written to by the computing system <NUM> (and thus which may be corrupt), there may be appropriate access control. But otherwise, concurrent access to the drive <NUM> as a whole is enabled.

The method <NUM> may also include mounting the drive to yet further computing system(s) only as a readable drive (and not as a writable drive) so as to be readable by the computing system(s). For instance, in <FIG>, the drive <NUM> is further mounted to the computing system <NUM> as represented by the arrow <NUM>. Thus, the first computing system <NUM>, the second computing system <NUM>, and the third computing system <NUM>, and potentially other computing systems (as represented by the ellipsis <NUM>) may have concurrent access to the drive <NUM>.

The drive has thereon a single data structure of fixed size. For instance, in <FIG>, the drive <NUM> has thereon a fixed-size data structure <NUM>. Even though user data may be written into the fixed-size data structure <NUM>, the state of the drive <NUM> would not change. This prevents corruption to the drive <NUM>, and makes the state of the drive <NUM> predictable and stable. The "state" of a drive is the data that is stored on the drive that is not the user data itself, but information used to manage the user data. Such state may also be referred to as "management data" or "non-user data". In the embodiment as claimed, the single data structure is a fixed-size log portion that is described further herein with respect to <FIG>.

This mounting of a single drive to multiple computing systems may be especially beneficial in the case of there being a primary computing system that is supported by one or more fallback secondary computing systems. This might be the case when the primary computing system is running a very important process for which backup is necessary by one or more secondary computing systems. It is common in a variety of cloud applications to have a primary computing system with multiple secondary computing systems. For instance, in <FIG>, computing system <NUM> may be a primary computing system, and each of computing systems <NUM> and <NUM> may be a secondary computing system.

If the primary computing system fails, failover occurs to one of the secondary computing systems, which will continue the process as a newly assigned primary computing system. This ensures that the process has sufficient redundancy so that the process is highly available. In this case, the primary computing system is operating upon the data, while the secondary computing systems read data preparing for the day they might be called upon to become the primary computing system.

<FIG> illustrates a method <NUM> for performing failover from a primary computing system to a secondary computing system when a failure of the primary computing system is detected. The method <NUM> is initiated upon detecting a failover event (act <NUM>). The failover event may be, for instance, that the primary computing system is no longer functioning at an expected level. If that failover event occurs, a secondary computing system is selected as a new primary computing system (act <NUM>). The drive mounted to that secondary computing system (i.e., the new primary computing system) is then updated (act <NUM>) by <NUM>) making the mount writable so that the new primary computing system may additionally write to the drive (act <NUM>), and/or <NUM>) notifying the new primary computing system that it is the primary computing system (act <NUM>). In one embodiment, secondary computing system(s) are each always able to write to the drive, but they exercise their own control to prevent themselves from so doing, unless they are notified that they are the new primary computing system.

If failover is necessary, one of the secondary computing systems may be selected as the primary, and the mounting of the drive to the secondary computing system may be augmented to a writable mounting also, or the secondary computing system may simply be notified that it is now the primary computing system. This significantly reduces the time required to perform failover since a drive need not now be mounted to the secondary computing system as part of the failover. The drive is already present, and available. The time required for a secondary computing system to assume the role of primary computing system may be reduced from a matter of minutes or seconds, to perhaps milliseconds. This improves the seemlessness in the failover process.

<FIG> illustrates a log environment <NUM> in which there is a log that is composed of two components - a fixed-size log portion <NUM> and a growable log portion <NUM>. The fixed-size log portion <NUM> is an example of the fixed-size data structure <NUM> that is the single file stored in the drive <NUM> of <FIG>. The fixed-size log portion is fixed in size as symbolically represented by the boundaries <NUM> and <NUM> being thicker. The fixed-size log portion <NUM> includes the more recent log entries recorded in the log (and includes the tail of the log). As will become apparent further below, an advantage of the fixed-size log portion <NUM> being constant in size is that readers of the fixed-size log portion <NUM> are able to deterministically know where a log entry is located within the fixed-size log portion <NUM> using only the log sequence number of that log record. Furthermore, the fixed-size log portion is in a format that is designed to be safely shared between readers and writers. This simplifies the process of enabling readers to read log entries from the log. Furthermore, the fixed size of the fixed-size log portion <NUM> allows it to be the fixed-size data structure <NUM> of <FIG>, so that user data may be written to the fixed-size log portion <NUM> without changing the non-user data on the drive <NUM>.

The fixed-size log portion <NUM> is sequentially written to as represented by the arrow <NUM>. When the end (e.g., end <NUM>) of the fixed-size log portion <NUM> is encountered, the writing wraps back (as represented by dashed-lined arrow <NUM>) to the beginning (e.g., beginning <NUM>) of the fixed-size log portion <NUM> to continue writing to the log. Thus, the writing to the fixed-size log portion <NUM> occurs in circular fashion. Because of the circular write pattern, older log entries will be overwritten by newer log entries. Prior to that happening, a destager component <NUM> writes those older log entries sequentially onto the end (e.g., end <NUM>) of the growable log portion <NUM>. In this manner, the growable log portion <NUM> grows sequentially in direction <NUM>. The growable log portion <NUM> is not stored within the drive <NUM>, since growing a log would change the state of the drive <NUM>, risking corruption when multiple computing systems are mounted to the drive <NUM>.

Thus, the fixed-size log portion <NUM> includes newer log entries, which are the log entries that are most often read. On the other hand, the growable log portion <NUM> includes older log entries that are less often read. Furthermore, the fixed-size log portion <NUM> will include the tail of the log, which is the last log entry written to the log as a whole. In case of failure, it is important to be able to identify the tail of the log since that log entry is the last log entry that there is a guaranty will be executed even if there is a failure that occurs prior to the data operation represented by the log entry having been completed. During recovery, the recovery process restores the last checkpoint, and redoes the data operations of each log entry one at a time until the tail of the log is encountered.

<FIG> illustrates a log environment 500A that represents a specific example of the log environment <NUM> of <FIG>. The fixed-size log portion 501A of <FIG> represents an example of the fixed-size log portion <NUM> of <FIG>. The growable log portion 502A of <FIG> represents an example of the growable log portion <NUM> of <FIG>. In this example, and in the claimed embodiment, the fixed-size log portion 501A is sized to include an odd number of sub-portions. In the claimed embodiment a sub-portion is a virtual log file. As will become apparent from the description below, an advantage of the log portion 501A being sized to include an odd number of virtual log files is that normal operation of initializing sub-portions is simplified, while still allowing recovery processes to find the tail of the log. In this specific example, the fixed-size log portion 501A is sized to include three virtual log files.

Suppose that thus far, the log is composed of <NUM> virtual log files (or "VLFs"), and that virtual log files are identified in sequential order as VLF1, VLF2, VLF3, and so forth. The fixed-size log portion 501A would include the last three virtual log files VLF19, VLF20, and VLF21. The older virtual log files VLF1 through VLF18 would have been previously destaged into the growable log portion 501A by the destager <NUM>.

In this example, and as most apparent from the fixed-size log portion 501A, each portion (e.g., virtual log file) includes a fixed number of blocks. While a virtual log file may typically have a very large number of blocks, to keep the example simple, each virtual log file (e.g., VLF19 to VLF21) is illustrated as having four (<NUM>) blocks. For instance, virtual log file VLF19 is sequentially composed of blocks 19A to 19D, virtual log file VLF20 is sequentially composed of blocks 20A to 20D, and virtual log file VLF21 is sequentially composed of blocks 21A to 21D.

When a log record is written into the persistent log, a block that includes that log record is written into the fixed-size log file. Each log record within the block occurs a slot within the block. A log sequence number may thus be composed of a concatenation of a virtual log file identifier, a block identifier, and a slot identifier. Note that with the knowledge that the fixed-size log portion 501A has within it VLF19, VLF20 and VLF21 sequentially arranged in that order, any reader can get any log record within any of those virtual files with just the log sequence number. The block can be identified from the log sequence number allowing the proper block to be read. Then, the slot identifier from the log sequence number may be used to extract the proper log record from that block.

In any case, blocks are written by the primary compute system <NUM> one block at a time sequentially to the fixed-size log file. In the example of <FIG>, a block is represented as written to when it has an asterisk in the upper right corner. Thus, at the point in time illustrated in <FIG>, the tail of the log (represented by pointer <NUM>) is just after the block 21A since block 21A is the last block written.

The destager <NUM> monitors the position of the tail of the log (i.e., the position of the most recent block written to) in the fixed-size log portion 501A, and ensures that any virtual log files that are about to be overwritten are destaged into the growable log portion 502A. Then, storage locations of the fixed-size log portion 501A that were used to store that newly-destaged virtual log file may be reused for a subsequent virtual log file.

<FIG> illustrates a log environment 500B that represents a subsequent state of the log environment 500B after virtual log file VLF19 has been destaged, and the storage locations of the fixed-size log file 501A (now called labelled 501B) reused by a subsequent virtual log file VLF22 having blocks 22A, 22B, 22C and 22D. In this example, the sub-portion identifier (e.g., the virtual log file identifier) for each successively initialized sub-portion (e.g., a virtual log file) is incremented each time a new sub-portion (e.g., a virtual log file) is initialized within the fixed-size log portion. Note also that the growable log portion 502B has now grown by one virtual log file to now include virtual log file VLF19. In this example, the tail of the log <NUM> in <FIG> has not moved compared to the tail of the log in <FIG>. That is, the tail of the log <NUM> is still just after block 21A.

Now suppose that a checkpoint is taken at this point (right after block 21A is written to the fixed-size log portion 501B). Then, consistent with the sequential and circular writing pattern suppose that the following blocks are then written to in sequence into the fixed-size log portion 501B: block 21B, block 21C, block 21D, block 22A, and block 22B. The result will be the log environment 500C of <FIG>. Note now that the fixed-size log portion 501C has up to block 22B shown with an asterisk, and thus the tail of the log <NUM> is now just after block 22B. Furthermore, for convenience, the snapshot pointer <NUM> is represented for the convenience of the reader and will be referenced further below.

Now suppose that a failure occurs when the tail of the log <NUM> is just after block 22B (as shown in <FIG>). The task during recovery would be to first restore the most recent snapshot. In <FIG>, that would bring the state of the data up to just after the data operation for all of the log records within block 21A are executed. But to bring the data fully current, the recovery process executes all subsequent data operations represented by subsequent log records until the tail of the log (at point <NUM>) is encountered. The problem though is identifying when the tail of the log is encountered. After all, the blocks 22C and 22D still have data in them, though it is the log records that were destaged as part of blocks 19C and 19D. Thus, it is important to recognize what blocks have been written to as part of the current virtual log file (VLF22), and distinguish those blocks from those blocks that have stale data from a prior circular write cycle.

In order to allow the recovery process to make this distinction, there is a new marker data within each block that, together with the current virtual log file identifier, allows the recovery process to deterministically conclude whether or not new data has been written to the block as part of the current virtual log file. In one embodiment, the new marker data may be two bits within a block. The value of these bits, in conjunction with the sub-portion identifier, allows the recover process to determine which blocks have been written to.

For instance, suppose that when writing to blocks of sub-portions having odd sub-portion identifiers (e.g. VLF19 or VLF <NUM>), the two bits are written with a first possible value (e.g., <NUM>). In that case, when writing to blocks of sub-portions having even sub-portion identifiers (e.g., VLF20 or VLF <NUM>), the two bits are written with the second possible value (e.g., <NUM>).

<FIG> is a log environment 500D that is the same as the environment 500C of <FIG>, except that an "O" fills those blocks that would have the bits <NUM> using the convention described in the previous paragraph, and that an "X" fills those blocks that would have the bits <NUM> using the convention described in the previous paragraph. The asterisks have been removed since they were used just for the convenience of the reader.

The sub-portion VLF20 has had all of its blocks written to (since the tail of the log is way forward in VLF22). Thus, because the sub-portion identifier VLF20 is even, the two bits would be <NUM>, and thus the blocks 20A, 20B, 20C and 20D are shown as having an X. Of course, recovery should look for the tail of the log in the most recent sub-portion VLF22. However, even though the recovery does not need to look for the tail of the log in any of the prior sub-portions, if the recovery did examine sub-portion VLF20, the recovery would know that the tail of the log is not there.

The sub-portion VLF21 has had all of its blocks written to (since the tail of the log is way forward in VLF22). Thus, because the sub-portion identifier VLF21 is odd, the two bits would be <NUM>, and thus the blocks 21A, 21B, 21C and 21D are shown as having an O. Thus, if the recovery did examine sub-portion VLF21, the recovery would know that the tail of the log is not there.

The sub-portion VLF22 has only some of its blocks written to since the tail of the log <NUM> is within the sub-portion VLF22. Specifically, because the sub-portion identifier VLF is even, the two bits of the two blocks 22A and 22B written to would be <NUM>, and are thus shown with the "X" inside. However, note that the old data from VLF <NUM> is still within blocks 22C and 22D. That old data was written to those blocks when the primary compute system was writing to blocks 19C and 19D as part of sub-portion VLF19. Since that data has not changed at all, the two bits of blocks 22C and 22D remain <NUM>, and are thus marked with an "O" inside. Thus, without having to reformat the storage space that was used for sub-portion VLF <NUM> when beginning to reuse the storage space for sub-portion VLF22, the recovery process may still find the tail of the log. This effect is achieved precisely because there are an odd number (three) of sub-portions within the fixed-size log portion, and the sub-portions are added with identifiers that alternate between even and odd (which happens when they monotonically increase by one each time).

Thus, the log written to the drive <NUM> may be optimized such that the more frequently accessed blocks (that include the most recent log records) are optimized so that any reader may read those blocks and access appropriate log records using only the log sequence number. Furthermore, the writes may happen during normal operation such that, in a subsequent recovery, the tail of the log may be quickly found, without having to expend effort reformatting the storage space in the fixed-size log portion. Thus, truly, the log service may acknowledge that the data operation is guaranteed to take place once the block having that log record is written into the log of the log environment <NUM> of <FIG>.

Additionally, because the fixed-size log portion is fixed in size, the management data of the drive <NUM> stays the same. Also, as described above, the fixed-size log portion is designed to be safely shared between readers and writers in that the content is self-describing. This allows readers and the writer to be safe from torn writes and other concurrent access distortions even though the readers and write need not communicate with each other. Thus, the drive <NUM> can be mounted to multiple computing systems.

Because the principles described herein operate in the context of a computing system, a computing system will be described with respect to <FIG>. Computing systems may, for example, be handheld devices, appliances, laptop computers, desktop computers, mainframes, distributed computing systems, datacenters, or even devices that have not conventionally been considered a computing system, such as wearables (e.g., glasses, watches, bands, and so forth). In this description and in the claims, the term "computing system" is defined broadly as including any device or system (or combination thereof) that includes at least one physical and tangible processor, and a physical and tangible memory capable of having thereon computer-executable instructions that may be executed by a processor.

As illustrated in <FIG>, in its most basic configuration, a computing system <NUM> typically includes at least one hardware processing unit <NUM> and memory <NUM>. The memory <NUM> may be physical system memory, which may be volatile, non-volatile, or some combination of the two. The term "memory" may also be used herein to refer to non-volatile mass storage such as physical storage media. If the computing system is distributed, the processing, memory and/or storage capability may be distributed as well.

The computing system <NUM> has thereon multiple structures often referred to as an "executable component". For instance, the memory <NUM> of the computing system <NUM> is illustrated as including executable component <NUM>. The term "executable component" is the name for a structure that is well understood to one of ordinary skill in the art in the field of computing as being a structure that can be software, hardware, or a combination thereof. For instance, when implemented in software, one of ordinary skill in the art would understand that the structure of an executable component may include software objects, routines, methods that may be executed on the computing system, whether such an executable component exists in the heap of a computing system, or whether the executable component exists on computer-readable storage media.

In such a case, one of ordinary skill in the art will recognize that the structure of the executable component exists on a computer-readable medium such that, when interpreted by one or more processors of a computing system (e.g., by a processor thread), the computing system is caused to perform a function. Such structure may be computer-readable directly by the processors (as is the case if the executable component were binary). Alternatively, the structure may be structured to be interpretable and/or compiled (whether in a single stage or in multiple stages) so as to generate such binary that is directly interpretable by the processors. Such an understanding of example structures of an executable component is well within the understanding of one of ordinary skill in the art of computing when using the term "executable component".

The term "executable component" is also well understood by one of ordinary skill as including structures that are implemented exclusively or near-exclusively in hardware, such as within a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), or any other specialized circuit. In this description, the term "component" or "vertex" may also be used. As used in this description and in the case, this term (regardless of whether the term is modified with one or more modifiers) is also intended to be synonymous with the term "executable component" or be specific types of such an "executable component", and thus also have a structure that is well understood by those of ordinary skill in the art of computing.

While not all computing systems require a user interface, in some embodiments, the computing system <NUM> includes a user interface <NUM> for use in interfacing with a user. The user interface <NUM> may include output mechanisms 612A as well as input mechanisms 612B. The principles described herein are not limited to the precise output mechanisms 612A or input mechanisms 612B as such will depend on the nature of the device. However, output mechanisms 612A might include, for instance, speakers, displays, tactile output, holograms, virtual reality, and so forth. Examples of input mechanisms 612B might include, for instance, microphones, touchscreens, holograms, virtual reality, cameras, keyboards, mouse of other pointer input, sensors of any type, and so forth.

Embodiments described herein may comprise or utilize a special purpose or general-purpose computing system including computer hardware, such as, for example, one or more processors and system memory, as discussed in greater detail below. Such computer-readable media can be any available media that can be accessed by a general purpose or special purpose computing system. Thus, by way of example, and not limitation, embodiments can comprise at least two distinctly different kinds of computer-readable media: storage media and transmission media.

Computer-readable storage media includes RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other physical and tangible storage medium which can be used to store desired program code means in the form of computer-executable instructions or data structures and which can be accessed by a general purpose or special purpose computing system.

A "network" is defined as one or more data links that enable the transport of electronic data between computing systems and/or components and/or other electronic devices. Transmissions media can include a network and/or data links which can be used to carry desired program code means in the form of computer-executable instructions or data structures and which can be accessed by a general purpose or special purpose computing system.

For example, computer-executable instructions or data structures received over a network or data link can be buffered in RAM within a network interface component (e.g., a "NIC"), and then eventually transferred to computing system RAM and/or to less volatile storage media at a computing system. Thus, it should be understood that readable media can be included in computing system components that also (or even primarily) utilize transmission media.

Computer-executable instructions comprise, for example, instructions and data which, when executed at a processor, cause a general-purpose computing system, special purpose computing system, or special purpose processing device to perform a certain function or group of functions.

Those skilled in the art will appreciate that the invention may be practiced in network computing environments with many types of computing system configurations, including, personal computers, desktop computers, laptop computers, message processors, hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, mobile telephones, PDAs, pagers, routers, switches, datacenters, wearables (such as glasses or watches) and the like. The invention may also be practiced in distributed system environments where local and remote computing systems, which are linked (either by hardwired data links, wireless data links, or by a combination of hardwired and wireless data links) through a network, both perform tasks. In a distributed system environment, program components may be located in both local and remote memory storage devices.

Those skilled in the art will also appreciate that the invention may be practiced in a cloud computing environment, which is supported by one or more datacenters or portions thereof.

For instance, cloud computing is currently employed in the marketplace so as to offer ubiquitous and convenient on-demand access to the shared pool of configurable computing resources. Furthermore, the shared pool of configurable computing resources can be rapidly provisioned via virtualization and released with low management effort or service provider interaction, and then scaled accordingly.

A cloud computing model can be composed of various characteristics such as on-demand, self-service, broad network access, resource pooling, rapid elasticity, measured service, and so forth. A cloud computing model may also come in the form of various application service models such as, for example, Software as a service ("SaaS"), Platform as a service ("PaaS"), and Infrastructure as a service ("IaaS"). The cloud computing model may also be deployed using different deployment models such as private cloud, community cloud, public cloud, hybrid cloud, and so forth. In this description and in the claims, a "cloud computing environment" is an environment in which cloud computing is employed.

Claim 1:
A method for mounting a drive, the method comprising:
mounting (<NUM>) the drive to a first computing system so as to be writable by the first computing system, the drive having thereon a single data structure of fixed size, such that a state of the drive does not change in response to the writing of user data into the single data structure of fixed size, wherein the state of the drive is data that is stored on the drive that is not the user data itself, but information used to manage the user data, and wherein the single data structure is a fixed-size log portion of a log, the fixed-size log portion being sized to include an odd number of virtual log files and each virtual log file including a fixed number of blocks; and
mounting (<NUM>) the drive to a second computing system so as to be readable, but not writable, by the second computing system, and so that the first computing system and the second computing system both have concurrent access to the drive.