On Dec 7, 2016, at 10:58 AM, Andreas Dilger <adilger@xxxxxxxxx> wrote: > > On Dec 7, 2016, at 2:52 AM, Renaud Mariana <rmariana@xxxxxxxxxx> wrote: >> >> Here are my answers, hope it will help solve this issue, thanks. >> >> Recap: >> dpkg kibana on ext4 over a nbd device takes 10 minutes >> with xfs it's only 30s. >> with ext4 no extends only 30s. >> >> >> kernels : >> 4.5.7 has this issue as older kernel like 4.4.34 >> The issue is also when nbd client & server run on same host >> >> >> How small are the files? >> here is the histogram of file sizes : http://pasteboard.co/6HC3nKyk2.png >> We can see 5000 files around 512 Bytes. > > Definitely there is no value to use fallocate for 512-byte files, or any > of the files that can be written in a single write() syscall. I'd expect > any reasonable tool to be using a write buffer of at least 2-4MB these > days to get good performance, so writes below the buffer size shouldn't > use fallocate() at all. > >> dpkg using fallocate() ? >> Yes, there are 16044 calls by the same process >> what are these uninitialized extents ? > > Uninitialized extents are preallocated ranges of a file on disk that will > read back as zero, but are not necessarily zero-filled at allocation time. > For large files that are written randomly (or written slowly and may have > contention from other writers) fallocate() + uninitialized extents will > preallocate the space for the file so that it is (largely) contiguous on > disk and overwrites will not result in random block allocation. It would probably be worthwhile to file a ticket in the Debian BTS: https://bugs.debian.org/cgi-bin/pkgreport.cgi?dist=unstable;package=dpkg with your information, so that there is something to submit a patch against. Cheers, Andreas
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