Date: Mon, 31 Mar 2008 14:21:44 -0600 From: Scott Long <scottl@samsco.org> To: Ivan Voras <ivoras@freebsd.org> Cc: freebsd-current@freebsd.org Subject: Re: Are large RAID stripe sizes useful with FreeBSD? Message-ID: <47F147D8.3030905@samsco.org> In-Reply-To: <fsr7fb$hl0$1@ger.gmane.org>
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Ivan Voras wrote: > Most of new hardware RAID controllers offer stripe sizes of 128K, 256K > and some also have 512K and 1M stripes. In the simplest case of RAID0 of > two drives, knowing that the data is striped across the drives and that > FreeBSD issues IO request of at most 64K, is it useful to set stripe > sizes to anything larger than 32K? I suppose something like TCQ would > help the situation but does anyone know how is this situation usually > handled on the RAID controllers? Large I/O sizes and large stripe sizes only benefit benchmarks and a narrow class of real-world applications. Large stripes have the potential to actually hurt RAID-5 performance since they make it much harder for the card to a full stripe replacement instead of a read-modify-xor-write. I hate to be all preachy and linux-like and tell you want you need or don't need, but in all honesty, large i/o's and stripes usually don't help typical filesystem-based mail/squid/mysql/apache server apps. I do have proof-of-concept patches to allow larger I/O's for selected controllers on 64-bit FreeBSD platforms, and I intend to clean up and commit those patches in the next few weeks (no, I'm not ready for nor looking for testers at this time, sorry). Scotthome | help
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