UFS2 snapshots on large filesystems
Eric Anderson
anderson at centtech.com
Sun Nov 6 17:46:28 PST 2005
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
--
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Eric Anderson Sr. Systems Administrator Centaur Technology
Anything that works is better than anything that doesn't.
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