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. -- Stan _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
