On 15/02/13 09:42, Chris Murphy wrote:

(digging back through some things now that the higher priority tasks
appear covered)


On Feb 8, 2013, at 2:42 PM, Stan Hoeppner [off-list ref]
wrote:

quoted

These tests use 4KB *aligned* IOs.

It seems SSD's commonly now are 8KB paged. [1] [2] At least on OS X
with a Samsung 830 SSD, I'm finding a meaningful difference between
alignment on 4K vs a 1M alignment. [3]

Sequential write and rewrite aren't affected. Sequential Input is
affected, 5.6% improvement by 8K aligning. Random Seeks see an 87%
improvement with 1M alignment. I haven't retested to see if an 8K
alignment produces as good a result as a 1M alignment. I haven't
tested the full effect of Bonnie++ chunk size which is 8KB by
default; but in all tests so far there's no meaningful difference
between a chunk size of 4KB and 8KB.

It's kind of annoying that SSD manufacturers aren't reporting a
"physical sector" mapped to the SSD page size; similar to how 512e AF
HDDs report 512 byte logical, 4096 byte physical sectors. The
implication of an SSD reporting a 512 byte physical sector is that
alignment doesn't matter. I think it might matter.

So assuming I don't mind wasting a few MB per disk (I was leaving empty
space at the end of the partition anyway), what would I need to instruct
fdisk and/or md to do to get the alignment right?

Current partition/disk is as follows:
fdisk -l /dev/sdc
Disk /dev/sdc: 480 GB, 480101368320 bytes
255 heads, 63 sectors/track, 58369 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1               1       58000   465884968   fd  Lnx RAID auto

fdisk -ul /dev/sdc:
Disk /dev/sdc: 480 GB, 480101368320 bytes
255 heads, 63 sectors/track, 58369 cylinders, total 937697985 sectors
Units = sectors of 1 * 512 = 512 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1              63   931769999   465884968   fd  Lnx RAID auto

Thanks,
Adam

-- 
Adam Goryachev
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