Josef “Jeff” Sipek

Today I was playing with blktrace, and graphing the results with seekwatcher. At one point, I ran acp (which is a lot like tar, but tries to be smarter) on a directory stored on an XFS volume, but I forgot that months ago, I created a sparse file 101PB (that’s peta) in size. Well, acp was happily reading all the sparse regions. I killed it, and decided to remove the gigantic file which was totally useless. About 30 seconds into the removal, I realized it would have been great to have a trace of that. Well, I started blktrace and about 12 minutes later the rm process finished.

I graphed it and here’s the result (click for larger version):

At first I was very confused why things looked the way they did, but eventually it dawned on me (after some discussion with Dave Chinner — XFS dude) that it’s all journal log traffic. I quickly ran xfs_info on the filesystem:

meta-data=/dev/sdb1              isize=256    agcount=16, agsize=1120031 blks
         =                       sectsz=512   attr=1
data     =                       bsize=4096   blocks=17920496, imaxpct=25
         =                       sunit=0      swidth=0 blks, unwritten=1
naming   =version 2              bsize=4096
log      =internal               bsize=4096   blocks=8750, version=1
         =                       sectsz=512   sunit=0 blks
realtime =none                   extsz=65536  blocks=0, rtextents=0

And things just made sense. I calculated the size of the log (see bolded numbers) to be (4096*8750) bytes, or 34.17 MB (base 2) or 35.84 MB (base 10). If you look at the graph, you’ll see that the disk offsets accessed were 35001 to 35035 MB or about 35MB! XFS puts the log near the middle of the disk to minimize seeks as much as possible, so as you may have guessed, my disk is about 70GB in size (it’s a U160 73GB SCSI disk).