On Tue, Dec 22, 2020 at 08:54:53AM +1100, Chris Dunlop wrote: > Hi, > > I have a 2T file fragmented into 841891 randomly placed extents. It takes > 4-6 minutes (depending on what else the filesystem is doing) to delete the > file. This is causing a timeout in the application doing the removal, and > hilarity ensues. > > The fragmentation is the result of reflinking bits and bobs from other files > into the subject file, so it's probably unavoidable. > > The file is sitting on XFS on LV on a raid6 comprising 6 x 5400 RPM HDD: > > # xfs_info /home > meta-data=/dev/mapper/vg00-home isize=512 agcount=32, agsize=244184192 blks > = sectsz=4096 attr=2, projid32bit=1 > = crc=1 finobt=1, sparse=1, rmapbt=1 > = reflink=1 > data = bsize=4096 blocks=7813893120, imaxpct=5 > = sunit=128 swidth=512 blks > naming =version 2 bsize=4096 ascii-ci=0, ftype=1 > log =internal log bsize=4096 blocks=521728, version=2 > = sectsz=4096 sunit=1 blks, lazy-count=1 > realtime =none extsz=4096 blocks=0, rtextents=0 > > I'm guessing the time taken to remove is not unreasonable given the speed of > the underlying storage and the amount of metadata involved. Does my guess > seem correct? > > I'd like to do some experimentation with a facsimile of this file, e.g. try > the remove on different storage subsystems, and/or with a external fast > journal etc., to see how they compare. > > What is the easiest way to recreate a similarly (or even better, > identically) fragmented file? > > One way would be to use xfs_metadump / xfs_mdrestore to create an entire > copy of the original filesystem, but I'd really prefer not taking the > original fs offline for the time required. I also don't have the space to > restore the whole fs but perhaps using lvmthin can address the restore > issue, at the cost of a slight(?) performance impact due to the extra layer. > Note that xfs_metadump doesn't include file data, only metadata, so it might actually be the most time and space efficient way to replicate the large file. You would need a similarly sized block device to restore to and would not be able to change filesystem geometry and whatnot. The former can be easily worked around by restoring the image to a file on a smaller fs though, which may or may not interfere with whatever performance testing you're doing. > Is it possible to using the output of xfs_bmap on the original file to drive > ...something, maybe xfs_io, to recreate the fragmentation? A naive test > using xfs_io pwrite didn't produce any fragmentation - unsurprisingly, given > the effort XFS puts into reducing fragmentation. > fstests has a helper program (xfstests-dev/src/punch-alternating) that helps create fragmented files. IIRC, you create a fully allocated file in advance and it will punch out alternating ranges based on the offset/size parameters. You might have to wait a bit for it to complete, but it's pretty easy to use (and you can always create a metadump image from the result for quicker restoration). Yet another option might be to try a write workload that attempts to defeat the allocator heuristics. For example, do direct I/O or falloc requests in reverse order and in small sizes across a file. xfs_io has a couple flags you can pass to pwrite (i.e., -B, -R) to make that easier, but that's more manual and you may have to play around with it to get the behavior you want. Brian > Cheers, > > Chris >
