On Fri, May 25, 2012 at 5:42 PM, Josef Bacik <josef@xxxxxxxxxx> wrote: > On Fri, May 25, 2012 at 05:35:37PM +0200, Alexander Block wrote: >> Hello, >> >> (this is a resend with proper CC for linux-fsdevel and linux-kernel) >> >> I would like to start a discussion on atime in Btrfs (and other >> filesystems with snapshot support). >> >> As atime is updated on every access of a file or directory, we get >> many changes to the trees in btrfs that as always trigger cow >> operations. This is no problem as long as the changed tree blocks are >> not shared by other subvolumes. Performance is also not a problem, no >> matter if shared or not (thanks to relatime which is the default). >> The problems start when someone starts to use snapshots. If you for >> example snapshot your root and continue working on your root, after >> some time big parts of the tree will be cowed and unshared. In the >> worst case, the whole tree gets unshared and thus takes up the double >> space. Normally, a user would expect to only use extra space for a >> tree if he changes something. >> A worst case scenario would be if someone took regular snapshots for >> backup purposes and later greps the contents of all snapshots to find >> a specific file. This would touch all inodes in all trees and thus >> make big parts of the trees unshared. >> >> relatime (which is the default) reduces this problem a little bit, as >> it by default only updates atime once a day. This means, if anyone >> wants to test this problem, mount with relatime disabled or change the >> system date before you try to update atime (that's the way i tested >> it). >> >> As a solution, I would suggest to make noatime the default for btrfs. >> I'm however not sure if it is allowed in linux to have different >> default mount options for different filesystem types. I know this >> discussion pops up every few years (last time it resulted in making >> relatime the default). But this is a special case for btrfs. atime is >> already bad on other filesystems, but it's much much worse in btrfs. >> > > Just mount with -o noatime, there's no chance of turning something like that on > by default since it will break some applications (notably mutt). Thanks, > > Josef I know about the discussions regarding compatibility with existing applications. The problem here is, that it is not only a compatibility problem. Having atime enabled by default, may give you ENOSPC for reasons that a normal user does not understand or expect. As a normal user, I would think: If I never change something, why does it then take up more space just by reading it? -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
