[zfs-discuss] Unlistable files

Andreas Dilger adilger at dilger.ca
Thu Apr 5 23:21:19 EDT 2018


On Apr 5, 2018, at 17:40, Gionatan Danti <g.danti at assyoma.it> wrote:
> 
> Il 05-04-2018 23:58 Andreas Dilger via zfs-discuss ha scritto:
>> It might also be that "ls" and ZFS directory iteration do not play well
>> together, skipping some entries in the directory (e.g. hash collisions,
>> or if telldir() and seekdir() do not work properly).  If your problem
>> only happens on large directories then this is a possibility.
> 
> Can you expand on that point? It would be quite concerning if basic call as telldir() and seekdir() do not work properly.

Well, *something* isn't working properly, it just isn't clear what that is. 

If the strace output does not show telldir() and seekdir() (or maybe just
seek(fd, SEEK_CUR, 0) on the directory), then that is not the issue. 

> How about the hash collision possibility? Are we speaking about the collision of *which* specific element (metadata, filename, etc)?

This can be an issue if telldir() returns a cookie to userspace, but the cookie
is not unique, then seekdir() finds the wrong entry to restart. There is logic
in ZFS to disambiguate non-unique cookies, but that could be broken. 

If the problem is easily reproduced, and did not exist in earlier releases,
then using "git bisect" to find the patch that added the problem is also
possible, if the problem is only with the directory iteration and is not a
bug in how the entry was inserted into the directory. 

Cheers, Andreas



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