ライトアンプリフィケーション

SSDにおける基本的処理

フラッシュメモリの特質として、メモリにあるデータを直接「書き換える」こと（オーバーライト）が原理上不可能であり、これはハードディスクドライブとは異なっている。SSDにデータが書き込まれるとき、該当するフラッシュメモリのセルはすべて「消去」されており、そのようなページ（通常サイズは4KiB）に対して一度にデータが書き込まれる。
The SSD controller on the SSD, which manages the flash memory and interfaces with the host system, uses a logical to physical mapping system known as logical block addressing (LBA) and that is part of the flash translation layer (FTL). 
When new data comes in replacing older data already written, the SSD controller will write the new data in a new location and update the logical mapping to point to the new physical location. The data in the old location is no longer valid, and will need to be erased before the location can be written again.
Flash memory can only be programmed and erased a limited number of times. This is often referred to as the maximum number of program/erase cycles (P/E cycles) it can sustain over the life of the flash memory. Single-level cell (SLC) flash, designed for higher performance and longer endurance, can typically operate between 50,000 and 100,000 cycles. 2011年現在, multi-level cell (MLC) flash is designed for lower cost applications and has a greatly reduced cycle count of typically between 3,000 and 5,000. A lower write amplification is more desirable, as it corresponds to a reduced number of P/E cycles on the flash memory and thereby to an increased SSD life.

SSDにおけるガベージコレクション(GC)

データは、複数の記憶セルで構成される「ページ」と言う単位でフラッシュメモリーに書き込まれる。しかし消去は、複数のページで構成される、より大きい「ブロック」と言う単位でのみ可能となる。もしあるブロック内のあるページのデータが不要になった場合（"stale"ページと呼ばれる）、そのブロック内の必要なデータがあるページのみ、他の消去済みブロックに転送（書込）される。 そして"stale"ページは転送（書込）されないため、転送先のブロックでフリーなページとして新しい別のデータを書き込むことができる。ここまでの処理を「ガベージコレクション(GC)」と呼ぶ。全てのSSDは何らかのGCの仕組みを備えているが、GCをいつ、どのように処理するかはそれぞれ異なっている。GCはSSDのライト・アンプリフィケーションに大きな影響がある。
読み込み動作については、フラッシュメモリーを消去する必要はないため、通常はライトアンプリフィケーションに関連付けられることはない。しかしリード・ディスターブ(en:Read_disturb)などの不良モードが発生する前に、そのブロックは書き直しが行われる。もっとも、この事は、ドライブのライトアンプリフィケーション対する実質的な影響度は低いと見られている。

バックグラウンドGC

GCのプロセスはフラッシュメモリーの読込と再書込動作を伴う。つまりホストからの新しい書込指令によって、1ブロック全体の読込と、そのブロックのうち有効なデータを含む部分の書込、そして新しいデータの書込がまず必要となる。これはシステムの性能を著しく低減させる。 
SSDのコントローラーには「バックグラウンドGC(BGC)」、「アイドル時GC(ITGC)」などと呼ばれる機能を備えるものがある。これは、コントローラーがSSDのアイドル時に、ホスト側から新しい書込データが来るよりも前に、フラッシュメモリーの複数のブロックを統合するものである。これによりデバイスの性能の低下を防ぐ事ができる。
If the controller were to background garbage collect all of the spare blocks before it was absolutely necessary, new data written from the host could be written without having to move any data in advance, letting the performance operate at its peak speed. The trade-off is that some of those blocks of data are actually not needed by the host and will eventually be deleted, but the OS did not tell the controller this information. The result is that the soon-to-be-deleted data is rewritten to another location in the flash memory increasing the write amplification. In some of the SSDs from OCZ the background garbage collection only clears up a small number of blocks then stops, thereby limiting the amount of excessive writes. Another solution is to have an efficient garbage collection system which can perform the necessary moves in parallel with the host writes. This solution is more effective in high write environments where the SSD is rarely idle. The SandForce SSD controllers and the systems from Violin Memory have this capability.

ファイルシステムに着目したGC

In 2010, some manufacturers (notably Samsung) introduced SSD controllers that extended the concept of BGC to analyze the file system used on the SSD, to identify recently deleted files and unpartitioned space. The manufacturer claimed that this would ensure that even systems (operating systems and SATA controller hardware) which do not support TRIM could achieve similar performance. The operation of the Samsung implementation appeared to assume and require an NTFS file system. It is not clear if this feature is still available in currently shipping SSDs from these manufacturers. Systematic data corruption has been reported on these drives if they are not formatted properly using MBR and NTFS.

TRIMコマンド

TRIM is a SATA command that enables the operating system to tell an SSD what blocks of previously saved data are no longer needed as a result of file deletions or using the format command. When an LBA is replaced by the OS, as with an overwrite of a file, the SSD knows that the original LBA can be marked as stale or invalid and it will not save those blocks during garbage collection. If the user or operating system erases a file (not just remove parts of it), the file will typically be marked for deletion, but the actual contents on the disk are never actually erased. Because of this, the SSD does not know the LBAs that the file previously occupied can be erased, so the SSD will keep garbage collecting them.
The introduction of the TRIM command resolves this problem for operating systems which support it like Windows 7, Mac OS (latest releases of Snow Leopard, Lion, and Mountain Lion, patched in some cases), and Linux since 2.6.33. When a file is permanently deleted or the drive is formatted, the OS sends the TRIM command along with the LBAs that are no longer containing valid data. This informs the SSD that the LBAs in use can be erased and reused. This reduces the LBAs needing to be moved during garbage collection. The result is the SSD will have more free space enabling lower write amplification and higher performance.

TRIMの制限と限界

The TRIM command also needs the support of the SSD. If the firmware in the SSD does not have support for the TRIM command, the LBAs received with the TRIM command will not be marked as invalid and the drive will continue to garbage collect the data assuming it is still valid. Only when the OS saves new data into those LBAs will the SSD know to mark the original LBA as invalid. SSD Manufacturers that did not originally build TRIM support into their drives can either offer a firmware upgrade to the user, or provide a separate utility that extracts the information on the invalid data from the OS and separately TRIMs the SSD. The benefit would only be realized after each run of that utility by the user. The user could set up that utility to run periodically in the background as an automatically scheduled task.
Just because an SSD supports the TRIM command does not necessarily mean it will be able to perform at top speed immediately after. The space which is freed up after the TRIM command may be random locations spread throughout the SSD. It will take a number of passes of writing data and garbage collecting before those spaces are consolidated to show improved performance.
Even after the OS and SSD are configured to support the TRIM command, other conditions will prevent any benefit from TRIM. As of early 2010, databases and RAID systems are not yet TRIM-aware and consequently will not know how to pass that information on to the SSD. In those cases the SSD will continue to save and garbage collect those blocks until the OS uses those LBAs for new writes.
The actual benefit of the TRIM command depends upon the free user space on the SSD. If the user capacity on the SSD was 100 GB and the user actually saved 95 GB of data to the drive, any TRIM operation would not add more than 5 GB of free space for garbage collection and wear leveling. In those situations, increasing the amount of over-provisioning by 5 GB would allow the SSD to have more consistent performance because it would always have the additional 5 GB of additional free space without having to wait for the TRIM command to come from the OS.

ユーザー空き領域

The SSD controller will use any free blocks on the SSD for garbage collection and wear leveling. The portion of the user capacity which is free from user data (either already TRIMed or never written in the first place) will look the same as over-provisioning space (until the user saves new data to the SSD). If the user only saves data consuming 1/2 of the total user capacity of the drive, the other half of the user capacity will look like additional over-provisioning (as long as the TRIM command is supported in the system).

Secure erase

The ATA Secure Erase command is designed to remove all user data from a drive. With an SSD without integrated encryption, this command will put the drive back to its original out-of-box state. This will initially restore its performance to the highest possible level and the best (lowest number) possible write amplification, but as soon as the drive starts garbage collecting again the performance and write amplification will start returning to the former levels. Many tools use the ATA Secure Erase command to reset the drive and provide a user interface as well. One free tool that is commonly referenced in the industry is called HDDErase. Parted Magic provides a free bootable Linux system of disk utilities including secure erase.
Drives which encrypt all writes on the fly can implement ATA Secure Erase in another way. They simply zeroize and generate a new random encryption key each time a secure erase is done. In this way the old data cannot be read anymore, as it cannot be decrypted. Some drives with an integrated encryption may require a TRIM command be sent to the drive to put the drive back to it original out-of-box state.

ウェアレベリング

If a particular block were programmed and erased repeatedly without writing to any other blocks, the one block would wear out before all the other blocks, thereby prematurely ending the life of the SSD. For this reason, SSD controllers use a technique called wear leveling to distribute writes as evenly as possible across all the flash blocks in the SSD. In a perfect scenario, this would enable every block to be written to its maximum life so they all fail at the same time. Unfortunately, the process to evenly distribute writes requires data previously written and not changing (cold data) to be moved, so that data which are changing more frequently (hot data) can be written into those blocks. Each time data are relocated without being changed by the host system, this increases the write amplification and thus reduces the life of the flash memory. The key is to find an optimum algorithm which maximizes them both.

静的データと動的データの分離

The separation of static and dynamic data to reduce write amplification is not a simple process for the SSD controller. The process requires the SSD controller to separate the LBAs with data which is constantly changing and requiring rewriting (dynamic data) from the LBAs with data which rarely changes and does not require any rewrites (static data). If the data is mixed in the same blocks, as with almost all systems today, any rewrites will require the SSD controller to garbage collect both the dynamic data (which caused the rewrite initially) and static data (which did not require any rewrite). Any garbage collection of data that would not have otherwise required moving will increase write amplification. Therefore separating the data will enable static data to stay at rest and if it never gets rewritten it will have the lowest possible write amplification for that data. The drawback to this process is that somehow the SSD controller must still find a way to wear level the static data because those blocks that never change will not get a chance to be written to their maximum P/E cycles.

シーケンシャルライト

When an SSD is writing data sequentially, the write amplification is equal to one meaning there is no write amplification. The reason is as the data is written, the entire block is filled sequentially with data related to the same file. If the OS determines that file is to be replaced or deleted, the entire block can be marked as invalid, and there is no need to read parts of it to garbage collect and rewrite into another block. It will only need to be erased, which is much easier and faster than the read-erase-modify-write process needed for randomly written data going through garbage collection.

ランダムライト

The peak random write performance on an SSD is driven by plenty of free blocks after the SSD is completely garbage collected, secure erased, 100% TRIMed, or newly installed. The maximum speed will depend upon the number of parallel flash channels connected to the SSD controller, the efficiency of the firmware, and the speed of the flash memory in writing to a page. During this phase the write amplification will be the best it can ever be for random writes and will be approaching one. Once the blocks are all written once, garbage collection will begin and the performance will be gated by the speed and efficiency of that process. Write amplification in this phase will increase to the highest levels the drive will experience.

性能への影響

The overall performance of an SSD is dependent upon a number of factors, including write amplification. Writing to a flash memory device takes longer than reading from it. An SSD generally uses multiple flash memory components connected in parallel to increase performance. If the SSD has a high write amplification, the controller will be required to write that many more times to the flash memory. This requires even more time to write the data from the host. An SSD with a low write amplification will not need to write as much data and can therefore be finished writing sooner than a drive with a high write amplification.

製品における状況

In September 2008, Intel announced the X25-M SATA SSD with a reported WA as low as 1.1. In April 2009, SandForce announced the SF-1000 SSD Processor family with a reported WA of 0.5 which appears to come from some form of data compression. Before this announcement, a write amplification of 1.0 was considered the lowest that could be attained with an SSD. Currently, only SandForce employs compression in its SSD controller.