On Mon, Aug 18, 2014 at 11:16:12AM -0500, Stan Hoeppner wrote: > On Mon, 18 Aug 2014 17:01:53 +1000, Dave Chinner <david@xxxxxxxxxxxxx> > wrote: > > On Sun, Aug 17, 2014 at 10:29:21PM -0500, Stan Hoeppner wrote: > >> Say I have a single 4TB disk in an md linear device. The md device has > a > >> filesystem on it formatted with defaults. It has 4 AGs, 0-3. I have > >> created 4 directories. Each should reside in a different AG, the first > >> in > >> AG0. Now I expand the linear device with an identical 4TB disk and > >> execute > >> xfs_growfs. I now have 4 more AGs, 4-7. I create 4 more directories. > >> > >> Will these 4 new dirs be created sequentially in AGs 4-7, or in the > first > >> 4 AGs? Is this deterministic, or is there any chance involved? On the > > > > Deterministic, assuming single threaded *file-system-wide* directory > > creation. Completely unpredictable under concurrent directory > > creations. See xfs_ialloc_ag_select/xfs_ialloc_next_ag. > > > > Note that the rotor used to select the next AG is set to > > zero at mount. > > > > i.e. single threaded behaviour at agcount = 4: > > > > dir number rotor value destination AG > > 1 0 0 > > 2 1 1 > > 3 2 2 > > 4 3 3 > > 5 0 0 > > 6 1 1 > > .... > > > > So, if you do what you suggest, and grow *after* the first 4 dirs > > are created, the above is what you'll get because the rotor goes > > back to zero on the fourth directory create. Now, with changing from > > 4 to 8 AGs after the first 4: > > > > dir number rotor value new inode location (AG) > > 1 0 0 > > 2 1 1 > > 3 2 2 > > 4 3 3 > > <grow to 8 AGs> > > 5 0 0 > > 6 1 1 > > 7 2 2 > > 8 3 3 > > 9 4 4 > > 10 5 5 > > 11 6 6 > > 13 7 7 > > 14 0 0 > > > >> real system these 4TB drives are actually 48TB LUNs. I'm after > >> deterministic parallel bandwidth to subsequently added RAIDs after each > >> grow operation by simply writing to the proper directory. > > > > Just create new directories and use the inode number to > > determine their location. If the directory is not in the correct AG, > > remove it and create a new one, until you have directories located > > in the AGs you want. > > > > Cheers, > > > > Dave. > > > Thanks for the info Dave. Was hoping it would be more straightforward. > Modifying the app for this is out of the question. They've spent 3+ years > developing with EXT4 and decided to try XFS at the last minute. Product is > to ship in October, so optimizations I can suggest are limited. Perhaps you could actually tell us what the requirement for layout/separation is, and how they are acheiving it with ext4. We really need a more "directed" allocation ability, but it's not clear exactly what requirements need to drive that. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
