Date: Sun, 06 Nov 2005 19:46:20 -0600 From: Eric Anderson <anderson@centtech.com> To: user <user@dhp.com> Cc: freebsd-fs@freebsd.org Subject: Re: UFS2 snapshots on large filesystems Message-ID: <436EB1EC.7010801@centtech.com> In-Reply-To: <436BDB99.5060907@samsco.org> References: <Pine.LNX.4.21.0511041531210.8180-100000@shell.dhp.com> <436BDB99.5060907@samsco.org>
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Scott Long wrote: > user wrote: > >> Hello, >> >> Considering a PC server running FreeBSD with 4 400 GB hard drives >> attached >> to a hardware raid controller doing raid-5. >> >> So this will present itself to the OS as a 1.2TB filesystem. >> >> Any comments on taking one or multiple snapshots of a filesystem of this >> size ? >> >> Given current disk capacities, I would not exactly consider this 1.2TB >> filesystem a "large" one ... any comments on say ... a 6-8 TB filesystem >> and making one or more snapshots of it ? >> >> Assume they are marginally busy - perhaps a 5-10% data turnover per >> day... >> >> Thanks. >> > > 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. I have several 2TB filesystems, which I do snapshots on. I can report that it indeed takes a long time to run, but works nevertheless. One thing to keep in mind though - fsck'ing a 2TB filesystem can take 2GB of memory (depends on how many files you have, and a few other factors). I have 4GB of memory in this box, and what I've seen so far is between 1.5GB and 2.5GB of memory required for fsck to run smoothly on a single partition. You definitely also want to turn off background fsck, unless you have extreme amounts of time on your hands. An fsck of a 70% full 2TB filesystem with a ton of files on it takes many hours, so I often mount the filesystems unclean (FreeBSD lets me do this) with rw, and continue my work until I can unmount, fsck, and remount the fs. This brings me to UFS Journaling - Scott, how's it coming along? I know you've been busy with the 6.0-RELEASE (great work by the way!!), but I'm itching for this. Is there anything I can do to help? Eric -- ------------------------------------------------------------------------ Eric Anderson Sr. Systems Administrator Centaur Technology Anything that works is better than anything that doesn't. ------------------------------------------------------------------------
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