1. Intel SSDs can do 90-100% of their sequential bandwidth with 16-32KB blocks @ QD 1, and at higher queue depths they can reach it at 8KB blocks. Harddisks on the other hand reach their maximum bandwidth around 64-128KB sequential blocks, and do not benefit noticably from increasing the queue depth.
When you RAID-0, the files that are larger than the stripe size get split up in chucks equal in size to the stripe size and distributed amongs the units in the RAID. Say you have a 128KB file (or want to read a 128KB chunk of a larger file), this will get divided into 8 pieces when the stripe size is 16KB, and with 3 SSDs in the RAID this means 3 chunks for 2 of the SSDs, and 2 chukcs for the third. When you read this file, you will read 16KB blocks from all 3 SSDs at Queue Depth 2 and 3. If you check out ATTO, you will see 2x 16KB @ QD 3 + 1x 16KB @ QD 2 summarize to higher bandwidth than 1x 128KB @ QD 1.
The bandwidth when reading or writing files equal to or smaller the stripe size will not be affected by the RAID. The sequential bandwidth of blocks of 1MB or larger will also be the same since the SSDs will be able to deliver max bandwidth with any stripe size since data is striped over all in blocks large enough or enough blocks to reach max bandwidth for each SSD.
So to summarize, benefits and drawbacks of using a small stripe size:
+ Higher performance of files/blocks above the stripe size while still relatively small (<1MB)
- Additional computational overhead from managing more blocks in-flight, although this is negligable for RAID-0.
The added performance of small-medium files/blocks from a small stripe size can make a difference for OS/apps, and can be meassured in PCmark Vantage.
2. Regarding the "Most SSD's have a native block size of 32KB when erasing......" quote, this is purely false.
Most SSDs have 4KB pages and 512KB erase-blocks. Anyways, as long as you have LBA->Physical block abstraction, dynamic wear leveling, and garbage collection, you can forget about erase-blocks and only think of pages.
This is true for Intels SSDs, and most newer SSD (2009 and newer).
These SSDs have "pools" of pre-erased blocks wich are written to, so you don't have to erase evertime you write. The garbage collection is responsible for cleaning dirty or partially dirty erase-blocks and combine them to pure valid blocks in new locations, and the old blocks then enter the "clean" pool.
Most SSDs are capable of writing faster than their garbage collection can clean, and therefore you get a lower "sustained" write speed than the max speed, it will however return back to max when the GC has had some time to replenish the clean pool. Some SSDs will sacrafice write amplification (by using more aggressive GC) to increase sustained sequential write.
Intel on the other hand has focused on maximizing the random write performance in a way that also minimizes write amplification, and this either means high temporary and really low sustained write, or like intel has done, fairly low sequential write that does not degrade much. (this has to do with write placement, wear leveling, and garbage collection)
This technique is what allows the x25-V to have random write equal to sequential write (or close to. 40MB/s random write, 45MB/s sequential write). x25-M could probably also get a random:seq write ratio close to 1:1, but the controller doesn't have enough computational power to deliver that high random write using intels technique.
3. Anyways, i thought i'd post it here so everyone could see:
The numbers he's refering to shows 16KB stripe as superior performance-wise.
Here's the PCmark vantage HDD scores of 3 x25-V's in RAID-0 by stripe size:
16KB: 74 164
32KB: 70 364
64KB: 63 710
128KB: 55 045
For those wondering, 16KB shows 540MB/s read and 131MB/s write in CrystalDiskMark 3.0 while 128KB shows 520MB/s read and 131MB/s write (1000MB lenght, 5 runs)
Also, here are the AS SSD total scores by stripe size for 3 x25-V's in RAID-0:
16KB: 809
32KB: 797
64KB: 795
128KB: 774
By doing PCmark vantage points multiplied by 2/3, i guess Anand used a 128KB stripe.
If he'd used a 16KB stripe, the numbers would likely be around 48-49 000
This is supported by benchmarking done by the user Anvil, who got 47 980 points in the Vantage HDD test with 2 x25-V's in RAID-0 off ICH10R with a 16KB stripe size. (IRST 9.6 driver, writeback cache disabled).
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Originally Posted by Osterman
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