ZFS UNMAP performance

[Danny Schales]

On 03/11/2014 10:48, Tom Evans wrote:
> On Tue, Mar 11, 2014 at 3:28 PM, Danny Schales <dan at latech.edu> wrote:
>> I'm seeing very slow performance with certain operations on a ZFS
>> filesystem built on ISCSI LUN's on a 10.0 system (new ISCSI
>> implementation).  The issue appears to be with BIO_DELETE operations.
>> Monitoring the system with gstat shows expected times for read and write
>> operations, but deletes are in the multiple hundreds of milliseconds
>> under normal operation.  Destroying a snapshot sends the times to
>> astronomical levels.  sysctl says the system is using UNMAP for deletes:
>>
>> kern.cam.da.0.delete_method: UNMAP
>>
>> I searched and found where Oracle issued a performance alert for Solaris
>> 11.1 where ZFS using UNMAP was in use.  Here's a link to a blog
>> discussing it:
>>
>> http://schalwad.blogspot.com/2013/12/solaris-111-zfs-write-performance.html
>>
>>
>> Is FreeBSD also impacted?  If so, is there a fix or a workaround?
> 
> FreeBSD is affected the same way Solaris is. IMHO, neither are
> affected, it is merely a consequence of using a device with a poor
> TRIM/UNMAP algorithm.
> 
> You can trivially tell ZFS to not use TRIM, or tweak the tunings to
> suit your devices. I'm afraid I don't know what they all mean
> precisely..
> 
> vfs.zfs.vdev.trim_on_init: 1
> vfs.zfs.vdev.trim_max_bytes: 2147483648
> vfs.zfs.vdev.trim_max_pending: 64
> vfs.zfs.trim.enabled: 1
> vfs.zfs.trim.txg_delay: 32
> vfs.zfs.trim.timeout: 30
> vfs.zfs.trim.max_interval: 1
> 
> Of course, if you disable TRIM you will end up with new (but
> different) poor performance characteristics. Probably.


The backend is an ISCSI LUN from a SAN (Dell Compellent).  From
research, it seems the SAN *does* support SCSI UNMAP requests for
deletes, but the performance is horrible.  I don't know if this is a
FreeBSD issue or a Compellent issue.  I haven't seen the problem with
other devices, but I don't think anything else is using UNMAP (yet).

Danny


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