From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Mason Subject: Re: parity data Date: Wed, 10 Sep 2008 08:59:29 -0400 Message-ID: <1221051569.29741.69.camel@think.oraclecorp.com> References: <1220766185.20018.7.camel@telesto> <1220885271.8537.31.camel@think.oraclecorp.com> <1220921206.7953.32.camel@telesto> <1220957012.29741.38.camel@think.oraclecorp.com> <1221010326.14596.9.camel@telesto> Mime-Version: 1.0 Content-Type: text/plain Cc: linux-btrfs To: Eric Anopolsky Return-path: In-Reply-To: <1221010326.14596.9.camel@telesto> List-ID: On Tue, 2008-09-09 at 19:32 -0600, Eric Anopolsky wrote: > > > Let's say I have 4 100GB drives (2 fast ones and 2 slow ones). I've > > > restricted a performance critical directory to the two fastest drives, > > > currently totaling 100GB of performance critical data. The rest of the > > > data on the system is striped. > > > > > > How much free space do I have on the filesystem? 100GB (the amount of > > > data I can store in the performance critical directory)? 200GB (the > > > amount of data I can store outside the performance critical directory if > > > the striping is guaranteed)? 300GB (the amount of data I can store > > > outside the performance critical directory if the striping is best > > > effort)? > > > > > > > People already create these configurations, they just do it with > > multiple filesystems. And, when they want to resize the performance > > critical section, it is a difficult (and often slow) operation. > > I think I'm starting to get it. btrfs would have drive groups, and no > file would have data on more than one drive group at once. That would > make it possible to make meaningful statements about how much free disk > space there is (per drive group). This is almost the same as having > multiple filesystems, except files cannot be assigned to filesystems on > an individual basis. Yes, I think this is a fair statement. > So in a way, btrfs would be replacing some > functionality of the VFS (mapping files to filesystems). I think there are many different definitions of the VFS. Mostly what the VFS does is maintain the dentry and inode caches, and provide a basic locking framework around most file/inode operations. The VFS is still doing all the mapping of files to filesystems, and the filesystem is mapping files to disk blocks. -chris