Date: Fri, 11 Nov 2005 11:28:59 -0600 From: Eric Anderson <anderson@centtech.com> To: user <user@dhp.com> Cc: freebsd-fs@freebsd.org Subject: Re: three follow-up questions RE: UFS2 snapshots on large filesystems Message-ID: <4374D4DB.7050109@centtech.com> In-Reply-To: <Pine.LNX.4.21.0511111150460.8180-100000@shell.dhp.com> References: <Pine.LNX.4.21.0511111150460.8180-100000@shell.dhp.com>
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user wrote: > > thank you scott - see below: > > On Fri, 4 Nov 2005, Scott Long wrote: > > >>The UFS snapshot code was written at a time when disks were typically >>around 4-9GB in size, not 400GB in size =-) Unfortunately, the amount >>of time it takes to do the initial snapshot bookkeeping scales linearly >>with the size of the drive, and many people have reported that it takes >>considerable amount of time (anywhere from several minutes to several >>dozen minutes) on large drives/arrays like you describe. So, you should >>test and plan accordingly if you are interested in using them. > > > > Testing is what I need to do. I have a few follow up questions: > > First, are there any sysctl or kernel tunables that change any of what you > are discussing above ? > > Second, let's say I am willing to accept the long snapshot creation period > ... are there other drawbacks as well during the course of _running with_ > the snapshot once it is created ? Or are all costs paid initially ? There will be a slight performance penalty with snapshots on the filesystem, but it shouldn't be noticeable. > Finally, I have read the bsdcon3 paper that mccusick wrote where he > addressed the dual problems of not enough kernel memory (10 megabytes) to > cache disk pages, and the system deadlocking that occurs with two > snapshots. Is it true that both of the fixes he elucidated in that paper > are built into what I see as fbsd 5.4 now ? I've seen deadlocking on 5.4, however some recent work on the filesystem and snapshots went into 6.0 and -CURRENT, and since then, I've seen no issues. I highly recommend 6.0 for this. Eric -- ------------------------------------------------------------------------ Eric Anderson Sr. Systems Administrator Centaur Technology Anything that works is better than anything that doesn't. ------------------------------------------------------------------------
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