A comment on snapshots in Linux LVM. About 6 months ago or so, I considered implementing LVM snapshots on a production RHEL4 box.
I decided to stress test the system by using a script to repeatedly create, delete and recreate snapshots for 100 iterations. After a while, the script hung and the filesystem was corrupted in an unrecoverable way, so I decided this was not yet ready for primetime. It's possible that all the bugs are now worked out, but IMHO I would stress test this extensively before using it for anything you really care about. Regards, Joshua Putnam Sr. Unix Administrator InterSystems Corporation 1 Memorial Drive Cambridge, MA 02142 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Tom Metro Sent: Thursday, August 23, 2007 4:30 PM To: L-bblisa Subject: Re: [BBLISA] filesystems supporting snapshots Peter Galvin wrote: > Or of course you can build a low cost NAS using ZFS - just use Solaris 10. True, if you happen to like Solaris. Robert Huff wrote: > Since ZFS is now available for FreeBSD, it is probably available for > Linux? As pointed out in a prior message, it is, but only as a user-space driver, due to licensing restrictions. The FreeBSD implementation is in-kernel. Adam Moskowitz wrote: > I'm pretty sure Kirk McKusick out snapshots in the FreeBSD FFS quite a > few years ago. I'd seen attempts at snapshot capability in open source projects before, but until it was pointed out in ZFS, I hadn't seen one that worked the way the NetApp filesystem does. (In my original comment, I should have been more specific that I meant NetApp-style snapshots.) For anyone not familiar with how the NetApp Write Anywhere File Layout (WAFL) file system works and how it accomplishes fast and space efficient snapshots, this technical paper describes WAFL on page 7, and snapshots on page 8: http://www.netapp.com/library/tr/3001.pdf A WAFL file system can be thought of as a tree of blocks rooted by a data structure that describes the inode file. The inode file in turn contains the inodes that describe the rest of the files in the system, including meta-data files such as the free block bitmap and the free inode bitmap. ... WAFL creates a new Snapshot copy by making a duplicate copy of the root data structure, as shown in Figure 1(b). Since the root data structure is only 192 bytes, and since no other data blocks need to be copied on disk, a new Snapshot copy does not actually consume any additional disk space until a user deletes or modifies data in the active file system. WAFL creates a Snapshot copy in just a few seconds. Figure 1(c) shows what happens when a user modifies data block C. WAFL writes the new data to block C' on disk, and changes the root structure for the active file system to point to the new block. The Snapshot copy still references the original block C, which is unmodified on disk. (Coincidentally, NetApp also has a paper on ZFS: http://www.netapp.com/library/tr/3603.pdf which can run on top of some of their hardware. The description of the snapshot capability is far more brief in that paper, but does include this statement, "Snapshot copies can be created almost instantly, and initially consume no additional disk space within the pool.") seph wrote: > linux's lvm has had snapshots for ages. I've noticed that, though have never tried that capability. I'm not sure how it is implemented, but given it is at a layer below the file system, I doubt it works with the speed and efficiency of the NetApp approach. Daniel Feenberg wrote: > We have had experience with FreeBSD snapshots, and found that taking a > snapshot took...typically several hours... > I posted a summary of our storage experiences at > http://www.nber.org/sys-admin/linux-nas-raid.html I realize you wrote that in response to Adam's comment above, referring to some existing snapshot capability in an existing FreeBSD file system, but to clarify for others, your referenced document ("last update 22 April 2006") isn't talking about ZFS on FreeBSD (which wasn't released until this year). -Tom -- Tom Metro Venture Logic, Newton, MA, USA "Enterprise solutions through open source." Professional Profile: http://tmetro.venturelogic.com/ _______________________________________________ bblisa mailing list [email protected] http://www.bblisa.org/mailman/listinfo/bblisa _______________________________________________ bblisa mailing list [email protected] http://www.bblisa.org/mailman/listinfo/bblisa
