Date: Tue, 24 Jan 2012 09:58:29 +0100 From: Alexander Leidinger <Alexander@Leidinger.net> To: Willem Jan Withagen <wjw@digiware.nl> Cc: freebsd-fs@freebsd.org Subject: Re: Question about ZFS with log and cache on SSD with GPT Message-ID: <20120124095829.Horde.cxpNZ5jmRSRPHnK1Ecp9wtA@webmail.leidinger.net> In-Reply-To: <4F1C3597.4040009@digiware.nl> References: <4F193D90.9020703@digiware.nl> <20120121162906.0000518c@unknown> <4F1B0177.8080909@digiware.nl> <20120121230616.00006267@unknown> <4F1BC493.10304@brockmann-consult.de> <4F1C3597.4040009@digiware.nl>
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Quoting Willem Jan Withagen <wjw@digiware.nl> (from Sun, 22 Jan 2012 17:13:11 +0100): > On 22-1-2012 9:10, Peter Maloney wrote: >> 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 > > Thanx for this thread, there is a lot of usefull info there. > pithy thing is to blow 66Mb, but then again on 40 or 120 Gb SSDs it is > only marginal. (Guess it stems from the time that HDs where 5Mb :) ) > > I'm still not really shure that that is needed it the bios has nothing > to do with these disks, as in our case: SSDs are only used as caches > under ZFS. I think the erase boundary only matters for speed, if the FS in question really deletes blocks in disk, instead of just "not using it anymore". I was told a while ago that ZFS is not doing BIO_DELETE, specially not on cache devices. So I do not expect that you will see an improvement by taking the erease boundary into account (except your SSD has not a decent wear-leveling implementation). Bye, Alexander. -- http://www.Leidinger.net Alexander @ Leidinger.net: PGP ID = B0063FE7 http://www.FreeBSD.org netchild @ FreeBSD.org : PGP ID = 72077137
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