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My SSD specifications say its peak read/write is 550/500 MB/s. What influences the actual speed? Is there any way to make a file that I need to be read fast to be always read at max speed?

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up vote 5 down vote accepted

SSDs are typically compared using four speed measurements:

  1. random read
  2. random write
  3. sequential read
  4. sequential write

The sequential speeds are usually higher than random speeds, because these are sustained reads or writes that can fully benefit from the fact that SSDs have several flash memory chips that can be accessed in parallel. Random reads and writes might only access one flash chip.

In your case, the manufacturer will be quoting optimal sequential speeds: using a large file, with no other operations at the same time.

Two points should be noted:

  • 550 MB/s will saturate a SATA 3.0 Gb/s bus; a newer SATA 6.0 Gb/s port is needed for the full benefit
  • Except for relatively infrequent tasks (such as moving large files to/from an SSD), it's usually more important to look at an SSD's random speeds, because these will generally have a more noticeable effect on day-to-day performance.
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I'm using SATA 6.0 Gbps. if you say sequential is faster, will defragmenting the drive frequently offer a large performance boost? as for my day-to-day performance, its mostly games and video converting. – Dani Aug 17 '11 at 16:36
@Dani Do not defragment an SSD - this will cause unnecessary wear of the flash memory, and will probably reduce performance over time. – sblair Aug 17 '11 at 16:40
that new to me. I can understand the wear part, but why would it reduce performance? – Dani Aug 17 '11 at 16:42
@Dani As the result of wear. Blocks that are "worn" will eventually be removed from the pool of usable space. Generally, the bigger the pool, the faster the SSD. Also, if TRIM is not enabled, there will be a much more immediate drop in write performance. – sblair Aug 18 '11 at 11:04
I don't know if trim is enabled. I have it connected to Z68 chipset SATA 6 Gbps controller with ACHI enabled. does that mean TRIM is on or I have to dig further in the BIOS? Also I have Marvell PCIe based SATA 6 Gbps controller, is there any chance moving the SSD on it will increase performance/lifetime? – Dani Aug 18 '11 at 13:50

You should not have to worry about anything on the drive side. On the OS side however, if you are separately reading other files, that could eat part of your 550 MB/s away.

As an aside, as this is something that you typically cannot change, your file will be read/written faster if they are compressable. Credit to @Col.

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Also some SSD's will only write at maximum speed for compressible files, I think it's the one's that use the Sandforce controllers – Col Aug 17 '11 at 16:04
He is talking about reading. – soandos Aug 17 '11 at 16:04
I just checked and it affects read speed as well, I wasn't sure about that. – Col Aug 17 '11 at 16:10
Thank you, added it to the answer. – soandos Aug 17 '11 at 16:12
@Col, mine uses sandforce, but with fast cpu speeds also hdd can read/write faster through gzip pipes and things like that. What about max entropy (uncompressible) data read/write? – Dani Aug 17 '11 at 16:32

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