Date: Mon, 13 Feb 2012 12:21:11 -0800 From: Adrian Chadd <adrian@freebsd.org> To: Alex Samorukov <ml@os2.kiev.ua> Cc: Harald Schmalzbauer <h.schmalzbauer@omnilan.de>, freebsd-stable@freebsd.org Subject: Re: disk devices speed is ugly Message-ID: <CAJ-VmokNuRSwD5pRj=vtJn4_Xoi4UN2Yxc1hV%2Be1XkpBC5b3zg@mail.gmail.com> In-Reply-To: <4F38AF69.6010506@os2.kiev.ua> References: <4F215A99.8020003@os2.kiev.ua> <4F27C04F.7020400@omnilan.de> <4F27C7C7.3060807@os2.kiev.ua> <CAJ-VmomezUWrEgxxmUEOhWnmLDohMAWRpSXmTR=n2y_LuizKJg@mail.gmail.com> <4F37F81E.7070100@os2.kiev.ua> <CAJ-Vmok9Ph1sgFCy6kNT4XR14grTLvG9M3JvT9eVBRjgqD%2BY9g@mail.gmail.com> <4F38AF69.6010506@os2.kiev.ua>
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I tend to say "the right solution to a problem is to not do it wrong." But.. given that Linux is fine with all the unaligned accesses, is the major sticking point here the fact that Linux's block dev layer is doing all the caching that FreeBSD's direct device layer isn't, and all of those (cached) accesses are what is improving performance? So perhaps it'd be worthwhile investing some time in a geom caching layer to see if that's at all feasible. I had the same problem with userland cyclic filesystems on FreeBSD versus Linux - the Linux FS performed better in synthetic tests because it did caching of the blockdev data. FreeBSD was doing direct IO. Tuning the direct IO sizes and fixing the filesystem code to do everything correctly aligned eliminated a lot of the ridiculous issues. Making Squid cache reads from disk would've improved it too. :-) Finally - I've seen this same issue under linux, especially when you stick a filesystem on a RAID device with the stripe/alignment all wrong. It's not just a BSD problem. :) Adrian
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