Date: Tue, 24 Aug 2021 11:30:38 +0000 From: bugzilla-noreply@freebsd.org To: bugs@FreeBSD.org Subject: [Bug 258022] [FUSES] Inode attributes are cached unnecessarily/for too long Message-ID: <bug-258022-227@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D258022 Bug ID: 258022 Summary: [FUSES] Inode attributes are cached unnecessarily/for too long Product: Base System Version: 13.0-RELEASE Hardware: Any OS: Any Status: New Severity: Affects Some People Priority: --- Component: kern Assignee: bugs@FreeBSD.org Reporter: chogata@moosefs.pro This is a problem a user of MooseFS reported. Under some circumstances crea= ting a new fs entry (any type: directory, regular file, special file) on MooseFS mount shows a message "Resource temporarily unavailable" and any subsequent operations on this inode (ls -al, rm or rmdir) also show this message. And MooseFS cannot be unmounted, the system shows a message "Device busy". Only= a restart of the whole machine helps. Since this was a bit similar to a problem some versions of Linux kernel had, when a process on one machine deleted a CWD of a process on a different machine, we at first thought it had to do with CWDs only and introduced some safeguards in MooseFS client for FreeBSD. But recent findings show this is = much more serious and on FreeBSD side. A simple test: we take two machines and mount MooseFS on both.=20 On FreeBSD machine (13.0-RELEASE-p3) we use these mount options: mfsmount -o mfsattrcacheto=3D0 -o mfsxattrcacheto=3D0 -o mfsentrycacheto=3D= 0 -o mfsdirentrycacheto=3D0 -o mfssymlinkcacheto=3D0 -o mfsgroupscacheto=3D0 /mn= t/mfs All the -o options are to disable any attribute caches that may exist (any lookup, access, mkdir etc. operations will return 0 seconds as cache time). We also have a second machine with the same MooseFS instance. Operating sys= tem on the second machine is irrelevant. Then we perform these steps, exactly in the order shown below: ***FreeBSD machine*** ~# cd /mnt/mfs/testdir /mnt/mfs/testdir# ls -al total 2932 drwxr-xr-x 2 root wheel 1 Aug 23 12:41 . drwxrwxrwx 43 root wheel 3001433 Aug 23 12:28 .. /mnt/mfs/testdir# ****** (FreeBSD "sees" that "testdir" is empty) ***OTHER machine*** ~# cd /mnt/mfs/testdir /mnt/mfs/testdir# mkdir dir /mnt/mfs/testdir# ****** (Other machine creates a directory named "dir" inside "testdir") ***FreeBSD machine*** /mnt/mfs/testdir# ls -al total 2933 drwxr-xr-x 3 root wheel 1 Aug 23 12:59 . drwxrwxrwx 43 root wheel 3001433 Aug 23 12:28 .. drwxr-xr-x 2 root wheel 1 Aug 23 12:59 dir /mnt/mfs/testdir# ****** (FreeBSD "sees" that there is now "dir" inside "testdir") ***OTHER machine*** /mnt/mfs/testdir# ls -ali total 2933 8 drwxr-xr-x 3 root wheel 1 Aug 23 12:59 . 1 drwxrwxrwx 43 root wheel 3001433 Aug 23 12:28 .. 9 drwxr-xr-x 2 root wheel 1 Aug 23 12:59 dir /mnt/mfs/testdir# rmdir dir /mnt/mfs/testdir# ls -al total 2932 drwxr-xr-x 2 root wheel 1 Aug 23 13:00 . drwxrwxrwx 43 root wheel 3001433 Aug 23 12:28 .. /mnt/mfs/testdir# ****** (We check the inode number of "dir" on the other machine and delete "dir") ***FreeBSD machine*** /mnt/mfs/testdir# ls -al total 2932 drwxr-xr-x 2 root wheel 1 Aug 23 13:00 . drwxrwxrwx 43 root wheel 3001433 Aug 23 12:28 .. /mnt/mfs/testdir# ****** (FreeBSD "sees" again, that "testdir" is empty) Now we wait for at least 5 minutes, the timing will be explained below. ***FreeBSD machine*** /mnt/mfs/testdir# echo "foo" > file.txt -bash: file.txt: Resource temporarily unavailable /mnt/mfs/testdir# ls -al ls: file.txt: Resource temporarily unavailable total 2932 drwxr-xr-x 2 root wheel 1 Aug 23 13:17 . drwxrwxrwx 43 root wheel 3001433 Aug 23 12:28 .. /mnt/mfs/testdir# ****** Ooops?! ***OTHER machine*** /mnt/mfs/testdir# ls -ali total 2932 8 drwxr-xr-x 2 root wheel 1 Aug 23 13:17 . 1 drwxrwxrwx 43 root wheel 3001433 Aug 23 12:28 .. 9 -rw-r--r-- 1 root wheel 0 Aug 23 13:17 file.txt /mnt/mfs/testdir# ****** The newly created file got the same inode number as the recently deleted directory "dir"... Notes: 1) The effect is not exclusive to former directory inode numbers becoming f= ile inode numbers. It happens whenever the new object is of a different type th= an the old one (so ex-directory inode number becomes re-used as file, ex-file = as fifo, ex-fifo as a device or directory etc.). The "ls -al" scenario is not = the only one, the same will happen if objects are created on FreeBSD machine and then deleted from another machine, which is of course a normal occurrence i= n a network file system. 2) Default inode reuse time in MooseFS is 24 hours. It was set to 5 minutes= for testing purposes only. The person, that reported the problem first (there w= ere others after), used the default 24 hours. And only inodes that are truly "f= ree" are reused, that means: no CWDs (active on any MooseFS client connected to = the instance), no sustained (deleted but still open) files are reused. The 24 h= our delay is counted from the moment they are considered free, so if a file is = in a sustained state for, let's say, 24 hours after deletion (and then whatever process had a hold on it finally finishes), its inode number is still not reused for another 24 hours. 3) Default cache times in MooseFS: file attributes cache timeout - 1 second, extended attributes (xattr) cache timeout - 30 seconds, directory entry cac= he timeout - 1 second, negative entry cache timeout - 0 seconds (default no negative cache), symbolic link cache timeout - 300 seconds, supplementary groups cache timeout - 300 seconds 4) Caches in the above experiment were ALL set to 0. 5) The problem was first reported on FreeBSD 12.1. So, to sum it up: we say "don't cache anything at all/longer than 300 secon= ds", FreeBSD caches indode attributes (we don't know, which ones, but at least t= ype) for longer than 24 hours and it causes a serious problem, because a new ino= de with reused inode number is basically unusable in the file system. --=20 You are receiving this mail because: You are the assignee for the bug.=
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