Re: [dm-devel] Barriers still not passing on simple dm devices...

[Ric Wheeler <rwheeler@redhat.com>]

Mikulas Patocka wrote:
>> And I will restate that back at EMC we tested the original barriers (with
>> reiserfs mostly, a bit on ext3 and ext2) and saw significant reduction in file
>> system integrity issues after power loss.
>>     
>
> You saw that barrier-enabled filesystem was worse than the same filesystem 
> without barriers? And what kind of issues were that? Disks writing damaged 
> sectors if powered-off in the middle of the writes? Or data corruptions 
> due to bugs in ReiserFS?
>   

No - I was not being clear. We saw a reduction in issues which is a 
confusing way to say that it was significantly better with barriers 
enabled, for both ext3 & reiserfs.

>   
>> The vantage point I had at EMC while testing and deploying the original
>> barrier work done by Jens and Chris was pretty unique - full ability to do
>> root cause failures of any component when really needed, a huge installed base
>> which could send information home on a regular basis about crashes/fsck
>> instances/etc and the ability (with customer permission) to dial into any box
>> and diagnose issues remotely. Not to mention access to drive vendors to
>> pressure them to make the flushes more robust. The application was also able
>> to validate that all acknowledged writes were consistent.
>>
>> Barriers do work as we have them, but as others have mentioned, it is not a
>> "free" win - fsync will actually move your data safely out to persistent
>> storage for a huge percentage of real users (including every ATA/S-ATA and SAS
>> drive I was able to test).  The file systems I monitored in production use
>> without barriers were much less reliable.
>>     
>
> With write cache or without write cache?
>   
Write cache enabled.

Barriers are off when write cache is disabled - we probe the drives 
write cache and enable barriers at mount time if and only if the 
barriers are on.
> With cache and without barriers the system is violating the specification. 
> There just could be data corruption ... and it will eventually happen.
>
> If you got corruption without cache and without barriers, there's a bug 
> and it needs to be investigated.
>
>   
>> As others have noted, some storage does not need barriers or flushed (high end
>> arrays, drives with no volatile write cache) and some need it but stink (low
>> cost USB flash sticks?) so warning is a good thing to do...
>>
>> ric
>>     
>
> Mikulas
>