Date: Wed, 31 Aug 2011 11:48:32 +0300 From: Daniel Kalchev <daniel@digsys.bg> To: freebsd-fs@freebsd.org Subject: Re: Very inconsistent (read) speed on UFS2 Message-ID: <4E5DF560.1050507@digsys.bg> In-Reply-To: <147623060.20110831123623@serebryakov.spb.ru> References: <1945418039.20110830231024@serebryakov.spb.ru> <317753422.20110830231815@serebryakov.spb.ru> <20110831004251.GA89979@icarus.home.lan> <147623060.20110831123623@serebryakov.spb.ru>
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On 31.08.11 11:36, Lev Serebryakov wrote: > device r/s w/s kr/s kw/s wait svc_t %b > ada1 340.9 292.9 43138.8 146.5 0 1.2 42 > ada2 340.9 293.9 43138.8 147.0 0 1.9 63 > ada3 340.9 292.9 43044.7 146.5 0 1.5 57 > ada4 341.9 292.9 43232.9 146.5 0 1.3 42 > ada5 341.9 292.0 43138.8 146.0 2 1.3 40 > Very interesting, this writes. You need to find out what is causing these. Just some random thoughts: This flapping may have something to do with the drives' internal caches. What are the drives? SATA drives, unlike SAS have simplex communication with the host, that is, the drive cannot simultaneously read and write data and commands (from/to host). There might be some, perhaps locking contention in there? It is not contention for bandwidth obviously. Most consumer drives have rather low IOPS performance. This is especially pronounced when there are both reads and writes. Your IOPS rate here is relatively high for such drive, although the busy percentage is low -- but then, it may not be accurate. In any case, you cannot measure read performance as long as it intermixes with writes, especially as you noted that your RAID5 code has some non-obvious write characteristics/optimizations. Daniel
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