Bug #23873: cephfs does not count st_nlink for directories correctly? - CephFS - Ceph

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Bug #23873

closed

cephfs does not count st_nlink for directories correctly?

Added by Danny Al-Gaaf about 6 years ago. Updated almost 6 years ago.

Status:

Resolved

Priority:

High

Assignee:

Patrick Donnelly

Category:

Correctness/Safety

Target version:

Ceph - v13.0.0

% Done:

Source:

other

Tags:

Backport:

luminous

Regression:

Severity:

3 - minor

Reviewed:

Affected Versions:

ceph-qa-suite:

Component(FS):

Client, kceph

Labels (FS):

Pull request ID:

Crash signature (v1):

Crash signature (v2):

Description

Not sure if this behavior is by intention, but if you create a empty directory on cephfs and call stat on the directory it returns '1' for st_nlink while other filesystems like XFS, EXT3/4 and even NFS return '2' for hard links.

How to reproduce:
1) mount cephfs
2) create directory on cephfs
3) call "stat" on the directory and check for "Links"
4) outcome: cephfs=1 but e.g. XFS=2

Tested with ceph version 12.2.1-361-gaea79b8b7a (aea79b8b7a65a8ca5cc1de84c7fe7d99eaa72ec0) luminous (stable)

Related issues 1 (0 open — 1 closed)

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Updated by Patrick Donnelly about 6 years ago

Category set to Correctness/Safety
Assignee set to Patrick Donnelly
Priority changed from Normal to High
Target version set to v13.0.0
Source set to other
Backport set to luminous
Component(FS) Client, kceph added

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Updated by Patrick Donnelly about 6 years ago

Status changed from New to Fix Under Review

https://github.com/ceph/ceph/pull/21652

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Updated by Zheng Yan about 6 years ago

If I remember right, this is not required by POSIX (btrfs does not do this). how NFS behaves depends on the exported filesystem. Besides, directory's nlink is not always 2 in EXT4/XFS. It is "2 + <number of subdirs>".

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Updated by Patrick Donnelly about 6 years ago

Zheng Yan wrote:

If I remember right, this is not required by POSIX (btrfs does not do this). how NFS behaves depends on the exported filesystem. Besides, directory's nlink is not always 2 in EXT4/XFS. It is "2 + <number of subdirs>".

Yes, I was thinking about that more today. "2 + <subdir count>" makes sense. hard-coded 2 definitely does not.

I didn't see anything in the spec about this either so I'm inclined to drop this. Danny, does this break something in particular or just an oddity?

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Updated by Peter Mauritius about 6 years ago

The Dovecot mail server does not work properly, if mailbox files are stored on cephfs and a mailbox prefix is configured. Dovecot is using a heuristic based on st_nlink to compute the number of subdirectories. The code assumes "2 + <subdir count>".

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Updated by Jeff Layton almost 6 years ago

Peter Mauritius wrote:

The Dovecot mail server does not work properly, if mailbox files are stored on cephfs and a mailbox prefix is configured. Dovecot is using a heuristic based on st_nlink to compute the number of subdirectories. The code assumes "2 + <subdir count>".

Then Dovecot is broken.

POSIX is mostly silent on the subject of st_nlink value for directories, and it can vary widely between filesystems. This is something that is really difficult to determine in a reliable way in many cases -- particularly on network filesystems.

I'd advise the Dovecot developers to re-think that heuristic. One should really not rely on the st_nlink value for directories to mean anything useful.

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