From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: Building a new raid6 with bitmap does not clear bits during resync Date: Wed, 14 Nov 2007 10:33:40 -0500 Message-ID: <473B1554.2050608@tmr.com> References: <87lk98ad7e.fsf@informatik.uni-tuebingen.de> <18231.62186.908021.981786@notabene.brown> <47387137.7000401@tmr.com> <18232.53770.923521.968923@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <18232.53770.923521.968923@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: Goswin von Brederlow , linux-raid@vger.kernel.org List-Id: linux-raid.ids Neil Brown wrote: > On Monday November 12, davidsen@tmr.com wrote: > >> Neil Brown wrote: >> >>> However there is value in regularly updating the bitmap, so add code >>> to periodically pause while all pending sync requests complete, then >>> update the bitmap. Doing this only every few seconds (the same as the >>> bitmap update time) does not notciable affect resync performance. >>> >>> >> I wonder if a minimum time and minimum number of stripes would be >> better. If a resync is going slowly because it's going over a slow link >> to iSCSI, nbd, or a box of cheap drives fed off a single USB port, just >> writing the updated bitmap may represent as much data as has been >> resynced in the time slice. >> >> Not a suggestion, but a request for your thoughts on that. >> > > Thanks for your thoughts. > Choosing how often to update the bitmap during a sync is certainly not > trivial. In different situations, different requirements might rule. > > I chose to base it on time, and particularly on the time we already > have for "how soon to write back clean bits to the bitmap" because it > is fairly easy to users to understand the implications (if I set the > time to 30 seconds, then I might have to repeat 30second of resync) > and it is already configurable (via the "--delay" option to --create > --bitmap). > Sounds right, that part of it is pretty user friendly. > Presumably if someone has a very slow system and wanted to use > bitmaps, they would set --delay relatively large to reduce the cost > and still provide significant benefits. This would effect both normal > clean-bit writeback and during-resync clean-bit-writeback. > > Hope that clarifies my approach. > Easy to implement and understand is always a strong point, and a user can make an informed decision. Thanks for the discussion. -- bill davidsen CTO TMR Associates, Inc Doing interesting things with small computers since 1979