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Optimizing Linux Systems For Solid State Disks
tytso writes "I've recently started exploring ways of configuring Solid State Disks (SSDs) so they work most efficiently in Linux. In particular, Intel's new 80GB X25-M, which has fallen down to a street price of around $400 and thus within my toy budget. It turns out that the Linux Storage Stack isn't set up well to align partitions and filesystems for use with SSD's, RAID systems, and 4k sector disks. There are also some interesting configuration and tuning that we need to do to avoid potential fragmentation problems with the current generation of Intel SSDs. I've figured out ways of addressing some of these issues, but it's clear that more work is needed to make this easy for mere mortals to efficiently use next generation storage devices with Linux."
> Vista has already started working around this problem, since it uses a default partitioning geometry of 240 heads and 63 sectors/track. This results in a cylinder boundary which is divisible by 8, and so the partitions (with the exception of the first, which is still misaligned unless you play some additional tricks) are 4k aligned. So this is one place where Vista is ahead of Linuxâ¦.
Although the technology it is used in is repugnant, NTFS has always been the One True Filesystem. It descended from DIGITAL's ODS2 (On Disk Structure 2) which traces back to the original Five Models (PDP 1, 8, 10, 11 and 12). You see, ODS was written by passionate people with degrees and rich personal lives in Massachusetts who sang and danced before the fall of humanity to the indignant Gates series who assimilated their young wherever possible and worked them into early graves during his epic battle with the Steves before the UNIX enemy remerged after a 25 year sleep and nuked the United States, draining all of its technological secrets to the other side of the world. Gates, realizing what he's done, now travels the universe seeking to rebuild his legacy by purifying humanity while the Steve series attempts to rebuild itself. Some of the original Five are still around, left to logon to Slashdot and witness what's left of the shadow of humanity still in the game as they struggle blindly around in epic circles indulging new and different ways to steal music, art and technology to make up for their lack of creativity long ago bred out of them by the Gates series.
Yeah, hard disk manufacturers.
Since they moved to large disks which require LBA, they've been fudging the CHS values returned by the drive to get the maximum size available to legacy operating systems. Since when did a disk have 63 heads? Never. It doesn't even make sense anymore when most hard disks are single platter (therefore having 1 or 2) and SSDs don't even have heads.
What they need to do is define a new command structure for accurately determining the best structure on the disk - on an SSD this would report the erase block size or so, on a hard disk, how many sectors are in a cylinder, without fucking around with some legacy value designed in the 1980's.
I don't think this is going to be a significant problem when compared to normal seek time problems.
Lets say we have 100 k of data to read. 512 byte blocks would require 200 reads. 4k blocks would require 25 reads.
For rotating discs: If the data is contiguous, we have to hope that all the blocks are on the same track. If they are, then there is 1 (potentially very costly) seek to get to the track with all the blocks on it. The cost of the seek is dependent on the track it's going to, the track it's on, and whether or not the drive is sleeping or spun down. Otherwise we also get to do another very short seek, which is going to add a bit of time to get to the next adjacent track. Worst case scenario all 200 blocks are on different tracks, scattered randomly on the platter, requiring 200 seeks. Ouch ouch ouch.
For SSDs: What is important is the number of cells we have to read. Cells will be 4k in size. All seek times are essentially zero. Best case scenario, all data is contiguous, and the start block is at the start of a cell. Read time boils down to how fast the flash can read 20 cells. Worst case scenario is where the data is 100% fragmented, such that all 200 512 byte blocks reside in a different cell, requiring 200 cell reads. (10fold increase in time required) There will also be overhead in copying out the 512 byte data from each buffer and assembling things, but this time is negligible for this comparison.
While the 20x time increase (order N) looks significant, it's important to compare the probabilities involved, and just how bad things get. The most important difference between how these two drives react is the space between fragments. In the "worse case' for SSD, 100% fragmentation, is highly unlikely. I don't even want to think about what a spinning disc would do if asked to perform a head seek for 100% of the blocks in say, a 1mb file. The read head would probably sing like a tuning fork at the very least. 2000 cell reads compared to 2000 seeks, the SSD will win handily every single time, even if the tracks on the disc are close.
If the spacing between fragments is anything near normal, say 30-100k, then there will be some seeking going on with the disc, and there will be some wasted cell reads with the SDD, but having to do an extra one cell read compared with having to do an extra head seek, again the SSD wins hands down. The advantage of the SSD actually goes down as fragmentation goes down, because most fragments are going to cause a head seek, each of will significantly widen the time gap. Also a spinning disc will read in the blocks much faster than the cells on a SSD.
I realize the OP was more describing the possibility of "not so much bang for the buck as you are expecting" due to fragmentation, and I know the above hits more on comparing the two than what happens to the SSD, but if you consider the effects of fragmentation on a spinning disc, and then weigh how the impact compares with a SSD, it's easy to see that fragmentation that sent you running for the defrag tool yesterday may not even be noticeable with a SSD. So I'd call this a "non-issue".
What I'm waiting for is them to invest the same dev time in read speeds as write speeds. SSDs don't appear to be doing any interleaved reads - they're doing it for the writes because they're so slow. Though at this point I wonder if read speeds are just plain running into a bus speed limit with the SSDs?
I work for the Department of Redundancy Department.
A real SSD has several advantages over using CF cards, but not for the reasons you state.
With a simple plug adapter, CF cards can be connected to an IDE interface, so speeds won't be limited by interface speed. The most recent revision of the CF spec adds support for IDE Ultra DMA 133 (133 MB/s)
A couple of additional points, just because I love nitpicking:
- A USB 2.0 mass storage device has a practical maximum speed of around 25 MB/s, not 40 Mb/s.
- The so-called SATA II interface (that name is actually incorrect and is not sanctioned by the standardization body) has a maximum speed of 300 MB/s, not Mb/s.