Date: Sun, 22 Jan 2012 09:10:59 +0100 From: Peter Maloney <peter.maloney@brockmann-consult.de> To: freebsd-fs@freebsd.org Subject: Re: Question about ZFS with log and cache on SSD with GPT Message-ID: <4F1BC493.10304@brockmann-consult.de> In-Reply-To: <20120121230616.00006267@unknown> References: <4F193D90.9020703@digiware.nl> <20120121162906.0000518c@unknown> <4F1B0177.8080909@digiware.nl> <20120121230616.00006267@unknown>
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Am 21.01.2012 23:06, schrieb Alexander Leidinger: >> Corsair reports: >> > Max Random 4k Write (using IOMeter 08): 50k IOPS (4k aligned) >> > So I guess that suggests 4k aligned is required. > Sounds like it is. > I'm not an SSD expert, but I read as much as I can, and found that many say that the sector size is not the only thing that matters on an SSD, but also the *erase boundary*. The size of the erase boundary varies, but 2MiB is a common factor (or 1MiB for 99% of them), so you can use that for all. The theory I read about is that when the SSD wants to write something, it must erase the whole erase block first. If it needs to erase a whole erase boundary space to write 512 bytes, that is just normal. But if you are misaligned, it often needs to erase 2 erase boundary spaces. Here is an example from our FreeBSD forum: http://forums.freebsd.org/showthread.php?t=19093 > I create the first partition at the usual 63 sectors offset from the > start of the disk (track 1) which is /unaligned/ with the SSD erase > block. The second partition is set to start at sector 21030912 > (10767826944 bytes) which is /aligned/ with the SSD erase block. > SSD erase block boundaries vary from manufacturer to manufacturer, but > a safe number to assume should be 1 MiB (1048576 bytes). In my testing, it made no difference. But as daniel mentioned: > With ZFS, the 'alignment' is on per-vdev -- therefore you will need to recreate the mirror vdevs again using gnop to make them 4k aligned. But I just resilvered to add my aligned disks and remove the old. If that applies to erase boundaries, then it might have hurt my test. Peterhome | help
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