Slashdot Mirror
Garbage Collection Algorithms Coming For SSDs
MojoKid writes "A common concern with the current crop of Solid State Drives is the performance penalty associated with block-rewriting. Flash memory is comprised
of cells that usually contain 4KB pages that are arranged in blocks of 512KB. When a cell is unused, data can be written to it relatively quickly. But if a cell already contains some data, even if it fills only a single page in the block, the entire block must be re-written. This means that whatever data is already present in the block must be read, then it must be combined or replaced, and the entire block is then re-written. This process takes much longer than simply writing data straight to an empty block. This isn't a concern on fresh, new SSDs, but over time, as files are written, moved, deleted, or replaced, many blocks are a left holding what is essentially orphaned or garbage data, and their long-term performance degrades because of it. To mitigate this problem, virtually all SSD manufacturers have incorporated, or soon will incorporate, garbage collection schemes into their SSD firmware which actively seek out and remove the garbage data. OCZ, in combination with Indilinx, is poised to release new firmware for their entire line-up of Vertex Series SSDs that performs active garbage collection while the drives are idle, in order to restore performance to like-new condition, even on a severely 'dirtied' drive."
This is the third generation, the second was to fix speed degradation through fragmentation.
And the fourth generation will fix SSD's small life longevity due to massive GC activity.
From the summary: "This isn't a concern on fresh, new SSDs, but over time, as files are written, moved, deleted, or replaced, many blocks are a left holding what is essentially orphaned or garbage data, and their long-term performance degrades because of it." The are talking about clearing sectors of garbage data that is no longer in use. It would have to be done anyways before the sector can be reused. The new firmware is simply doing that time consuming step early while it is in idle. The actual number of write cycles is not changing.
There is an extensions that was recently added to ATA, the TRIM command. The TRIM command allows an OS to specify a blocks data is no longer useful and the drive should dispose of it. No productions support it, but several beta firmwares do. There are also patches for the Linux kernel that adds support to the black layer along with appropriate support to most filesystems. Windows 7 also has support for it.
There is a lot of confusion about this on the OCZ boards, with people thinking GC somehow magically obviates the needs for TRIM. As you pointed out the GC doesn't know what is data and what is not with respect to deleted files in the FS. I wrote a blog post (with pictures and everything) explaining this just a few days ago
So, I delete a file off of a drive such that the Filesystem no longer holds any references to the given data, and the firmware moves in and performs operations to improve the performance of the device. Its not really rearranging files in to contiguous sections like defragmentation does, its restoring unused sections to an empty state, probably using an algorithm similar to many garbage collectors -- sounds like garbage collection to me.
Simple. Well, not really, but...
SSD's can be written to in small increments, but can only be erased in larger increments. So, you've got a really tiny pencil lead that can write data or scribble an "X" in an area to say the data is no longer valid, but a huge eraser that can only erase good-sized areas at a time, but you can't re-write on an area until it's been erased. There's a good explanation for this that involves addressing and pinouts of flash chips, but I'm going to skip it to keep the explanation simple. Little pencil lead, big eraser.
Let's call the small increment (what you can write to) a "block" and the larger increment (what you can erase) a "chunk". There are, say, 512 "blocks" to a "chunk".
So when a small amount of data is changed, the drive writes the changed data to a new block, then marks the old block as "unused". When all the blocks in a chunk are unused, the entire chunk can then be safely wiped clean. Until that happens, if you erase a chunk, you lose some data. So as time goes on, each chunk will tend to be a mix of current data, obsolete data, and empty blocks that can still be written to. Eventually, you'll end up with all obsolete data in each chunk, and you can wipe it.
However, it's going to be rare that ALL the blocks in a chunk get marked as unused. For the most part, there will be some more static data (beginnings of files, OS files, etc) that changes less, and some dynamic data (endings of files, swap/temp files, frequently-edited stuff) that changes more. You can't reasonably predict which parts are which, even if the OS was aware of the architecture of the disc, because a lot of things change on drives. So you end up with a bunch of chunks that have some good data and some obsolete data. The blocks are clearly marked, but you can't write on an obsolete block without erasing it, and you can't erase a single block - you have to erase the whole chunk.
To fix this, SSD drives take all the "good" (current) data out of a bunch of partly-used chunks and write it to a new chunk or set of chunks, then marks the originals as obsolete. The data is safe, and it's been consolidated so there are fewer unusable blocks on the drive. Nifty, except...
You can only erase each chunk a certain number of times before it dies. Flash memory tolerates reads VERY well. Erases, not so much.
So if you spend all of your time optimizing the drive, you're moving data around unnecessarily and doing a LOT of extra erases, shortening the hard drive's life.
But if you wait until you are running low on free blocks before you start freeing up space (which maximizes the lifespan of the drive), you'll run into severe slowdowns where the drive has to make room for the data you want to write, even if the drive is sitting there almost empty from the user's perspective.
So, SSD design has to balance between keeping the drive as clean and fast as possible at a cost of drive life, or making the drive last as long as possible but not performing at peak all the time.
There are certain things you can do to benefit both, such as putting really static data into complete chunks where it's less likely to be mixed with extremely dynamic data. But overall, the designer has to choose somewhere on the continuum of "lasts a long time" and "runs really fast".
"This post contains words, known to the State of California to cause thought. Wash brain thoroughly after reading."
I have been working closely with OCZ on this new firmware and wanted to clear things up a bit. This new firmware *does not*, *in any way at all*, remove or eliminate orphaned data, deleted files, or anything of the like. It does not reach into the partition $bitmap and figure out what clusters are unused (like newer Samsung firmwares). It does not even use Windows 7 TRIM to purge unused LBA remap table entries upon file deletions.
What it *does* do is re-arrange in-place data that was previously write-combined (i.e. by earlier small random writes taking place). If data was written to every LBA of the drive, then all files were subsequently deleted, all data would remain associated with those LBAs. This actually puts OCZ above most of the pack, because their algorithm restores performance without needing to reclaim unused flash blocks, and does so completely independent of the data / partition type used. This is particularly useful for those concerned with data recovery of deleted files, since the data is never purged or TRIMmed.
Slashdot-specific Translation: This firmware will enable an OCZ Vertex to maintain full speed (~160 MB/sec) sequential writes and good IOPS performance when used under Mac and Linux.
Hardware-nut Translation: This firmware will enable OCZ Vertex to maintain full performance when used in RAID configurations.
I'll have my full evaluation of this firmware up at PC Perspective later today. Once available, it will appear at this link:
http://www.pcper.com/article.php?aid=760
Regards,
Allyn Malventano
Storage Editor, PC Perspective
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I think you're somewhat close, but there are some inaccuracies...
Block devices (typically HD's) have two operations (read and write). These operations are what most modern operating system use. Flash SSD's emulate a block device, but the underlying flash memory uses three operations (read, write, and erase). The main difference, therefore, between the block device (what the OS references) and the underlying flash itself is the extra erase operation.
To write to a flash drive, assuming a cell has already been erased, all a user must do is a write operation. This operation is typically fast and does not affect the lifespan of the flash. A write can change any or all of the bits in a block from 1 to 0. Once this is complete, the requested data is written. However, if a user wants to overwrite or change existing data, they must first perform a block erase. This sets every bit in the block back to 1, and is typically very slow (compared to a write). In addition, this is what wears out the flash block, so we really want to avoid these operations.
Since flash blocks each have their own lifespan, we want to spread the erase operations around the disk. This is called wear leveling. To do this, the flash device appears like a block device to the operating system, but it remaps where the data is actually located at the physical flash layer with a remap table. For instance, let's say you overwrite a block in Linux. If there is an available free flash block, it may not even overwrite that block--it may allocate a new block for the file and write it there (updating the remap table). This avoids an erase command. Furthermore, there are a few files on a filesystem which change frequently, and if we did not move their location around the physical flash, we would wear out one cell in flash extremely quickly, even though the remainder of the cells had plenty of life left.
The garbage collection comes in due to this remapping. Typical block sizes for most OS filesystems are around 4k, but flash blocks are typically 512KB in flash devices. This means that if you send data to a SSD, it may or may not take up an entire page, as you may only be using 4k of actual data. Eventually, as writes are leveled around the drive and often fragmented (as we may not be occupying the entire 512KB block), future writes begin taking one (or more erase cycles). For instance, if you request that 512KB of data be written to the drive, but all the cells in the flash are physically occupied by a small amount of data, then data from multiple cells must be combined into one cell (multiple reads+erase+write), and then the destination cell that you are writing to must also be erased and written. This is what causes flash SSD's performance to significantly degrade over time.
By performing this recombining in the background (as a garbage collection), this should allow flash SSD's to maintain like-new performance even when containing a lot of data. In essence, they are performing background defragmentation on the SSD. As a sidenote, NEVER defragment an SSD from the Operating System, as this defragments the filesystem, but performs a ton of erase+write operations to the flash. At best, on new SSDs (Intel, Indilinx), this will wear out the drive sooner. On old SSD's, this will also increase fragmentation at the flash remap layer, causing further performance loss.
So to address your initial comment, rewrites would also see a performance increase by this garbage collection, as "rewriting" data in flash is virtually equivalent to a new write, since the remap table essentially moves the data anyway.
Source:
http://en.wikipedia.org/wiki/Flash_memory#Block_erasure
-=Lothsahn=-