Re: [PATCH] issue storage device flush via sync_blockdev() (was Re: Linux 2.6.29)

Jeff Garzik
2009-03-26T01:27:21Z

Ric Wheeler wrote:
> Eric Sandeen wrote:
>> Jeff Garzik wrote:
>>
>>> On Wed, Mar 25, 2009 at 01:40:37PM -0700, Linus Torvalds wrote:
>>>
>>>> On Wed, 25 Mar 2009, Jeff Garzik wrote:
>>>>
>>>>> It is clearly possible to implement an fsync(2) that causes FLUSH
>>>>> CACHE to be
>>>>> issued, without adding full barrier support to a filesystem. It is
>>>>> likely
>>>>> doable to avoid touching per-filesystem code at all, if we issue
>>>>> the flush
>>>>> from a generic fsync(2) code path in the kernel.
>>>>>
>>>> We could easily do that. It would even work for most cases. The
>>>> problematic ones are where filesystems do their own disk management,
>>>> but I guess those people can do their own fsync() management too.
>>>>
>>>> Somebody send me the patch, we can try it out.
>>>>
>>> This is a simple step that would cover a lot of cases... sync(2)
>>> calls sync_blockdev(), and many filesystems do as well via the generic
>>> filesystem helper file_fsync (fs/sync.c).
>>>
>>> XFS code calls sync_blockdev() a "big hammer", so I hope my patch
>>> follows with known practice.
>>>
>>> Looking over every use of sync_blockdev(), its most frequent use is
>>> through fsync(2), for the selected filesystems that use the generic
>>> file_fsync helper.
>>>
>>> Most callers of sync_blockdev() in the kernel do so infrequently,
>>> when removing and invalidating volumes (MD) or storing the superblock
>>> prior to release (put_super) in some filesystems.
>>>
>>> Compile-tested only, of course :) But it should be work :)
>>>
>>> My main concern is some hidden area that calls sync_blockdev() with
>>> a high-enough frequency that the performance hit is bad.
>>>
>>> Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
>>>
>>> diff --git a/fs/buffer.c b/fs/buffer.c
>>> index 891e1c7..7b9f74a 100644
>>> --- a/fs/buffer.c
>>> +++ b/fs/buffer.c
>>> @@ -173,9 +173,14 @@ int sync_blockdev(struct block_device *bdev)
>>> {
>>> int ret = 0;
>>>
>>> - if (bdev)
>>> - ret = filemap_write_and_wait(bdev->bd_inode->i_mapping);
>>> - return ret;
>>> + if (!bdev)
>>> + return 0;
>>> +
>>> + ret = filemap_write_and_wait(bdev->bd_inode->i_mapping);
>>> + if (ret)
>>> + return ret;
>>> +
>>> + return blkdev_issue_flush(bdev, NULL);
>>> }
>>> EXPORT_SYMBOL(sync_blockdev);
>>>
>>
>> What about when you're running over a big raid device with
>> battery-backed cache, and you trust the cache as much as much as the
>> disks. Wouldn't this unconditional cache flush be painful there on any
>> of the callers even if they're rare? (fs unmounts, freezes, unmounts,
>> etc? Or a fat filesystem on that device doing an fsync?)
>>
>> xfs, reiserfs, ext4 all avoid the blkdev flush on fsync if barriers are
>> not enabled, I think for that reason...
>>
>> (I'm assuming these raid devices still honor a cache flush request even
>> if they're battery-backed? I dunno.)
>>
>> -Eric
>>
> I think that Jeff's patch misses the whole need to protect transactions,
> including meta data, in a precise way. Useful for thing like unmount,
> not to give us strong protection for transactions or for fsync().

What do you think sync_blockdev() does? What is its purpose?

Twofold:
(1) guarantee all user data is flushed out before a major event
(unmount, journal close, unplug, poweroff, explosion, ...)

(2) As a sledgehammer hack for simple or legacy filesystems that do not
wish or need the complexity of transactional protection.
sync_blockdev() is intentionally used in lieu of complexity for the
following filesystems: HFS, HFS+, ADFS, AFFS, FAT, bfs, UFS, NTFS, qnx4.

My patch adds needed guarantees, only for the above filesystems, where
none were present before.

> This patch will be adding overhead here - you will still need flushing
> at the transaction commit layer of the specific file systems to get any
> reliable transactions.

sync_blockdev() is used as fsync(2) only in simple or legacy filesystems
that do not want a transaction commit layer!

Read the patch :)

Jeff