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Dieter.Soltau
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PostPosted: Fri Nov 09, 2012 8:47 pm    Post subject: bad SATA performance.. Reply with quote

i am on AMD64 (if that matters) and today i bought an SSD (Samsung 840 .. fairly new). i just get about 50MB/s ?? SATA3 controller installed.. reading from sata2 hdd..

now i checked and i copied from sata 2 to sata3 (non ssd) and i only get 12MB/s .. so i am just curious what is going wrong.. why this low performance?

this is a dualboot, and a benchmark on win7 runs fine.. 400MB/s read on the SSD..

the SATA3 controller is external.. ASUS U3S6 .. because my board just supports SATA2. the card is plugged into PCIE 4x .. as it should be. also plugging it into PCIE 16x makes no difference..

is thisa driver problem.. ?? the card has a marvel chipset.. AHCI is set everyhwere.. hmm..

i have to clue where i should start looking first?

btw.. i get the same bad performance if i copy using SATA drives using the onboard controler i just checked.. so it is not the addon cards fault..
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NeddySeagoon
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PostPosted: Fri Nov 09, 2012 9:03 pm    Post subject: Reply with quote

Dieter.Soltau,

You hint that the drive is used. Has it been used with a filesystem that supports trim?
If not, when discarded space is reused, the write speed will be slow as the drive has to run erase before it can write. Erase operations are very slow.
trim allows the erase to be performed as soon as the discarded blocks are no longer in use. The speeds up future write operations. Read speeds are not affected.

mkfs should have run a discard operation if you made new filesystems, which trims all of the blocks allocated to the filesystem.

The other option is the mix of SATA interface speeds - do you see any error messages in dmesg about SATA resets?

Partition layout is also important. SSDs use 4k, or bigger physical blocks. Its important that your partitions are aligned to physical disk blocks.
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NeddySeagoon

Computer users fall into two groups:-
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Dieter.Soltau
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PostPosted: Fri Nov 09, 2012 9:06 pm    Post subject: Reply with quote

yes it supports TRIM, and TRIM does actually work..
i fear a driver problem..
i used mkfs.ext4 -E discard..

TRIM is not the prob, the drive is fresh formatted and empty..
also the "normal" sata2 disks are slow :(

maybe since i switched to BIOS to AHCI..

wil check with dmesg and report back, thx

//edit= is ext4 using 4k blocks by default? i guess the blocksize of the SSD is 4K.. the technical datasheet does not tell :( it is an samsung 840.. the 250GB one (not the pro) and no, no errors in dmesg, hmm..

//edit2: duh! no AHCI support compiled in kernel.. now i am at 115MB/s writespeed.. but still a little slow IMHO.. compared to win7..
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Ant P.
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PostPosted: Fri Nov 09, 2012 9:35 pm    Post subject: Reply with quote

Some SSDs have a "4k-unaligned access" counter in the SMART data. That might give a hint.
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Dieter.Soltau
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PostPosted: Fri Nov 09, 2012 9:54 pm    Post subject: Reply with quote

Ant P. wrote:
Some SSDs have a "4k-unaligned access" counter in the SMART data. That might give a hint.

pls explain.. btw.. i am talking about this drive:

http://www.samsung.com/de/consumer/memory-storage/ssd/ssd-840/MZ-7TD250KW-spec

//edit: so more i copy to the drive, so slower it gets.. maybe thats normal? specially on plenty small files..
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