[stgt] [PATCH 1/1] Readonly disk LUNs
ronniesahlberg at gmail.com
Fri Mar 12 03:49:02 CET 2010
On Fri, Mar 12, 2010 at 12:57 PM, FUJITA Tomonori
<fujita.tomonori at lab.ntt.co.jp> wrote:
> On Tue, 9 Mar 2010 17:00:50 +1100
> ronnie sahlberg <ronniesahlberg at gmail.com> wrote:
>> Ok, initially it was triggered by the other thread bout reaonly for
>> some initiators.
>> That is probably a very bad example since there are other issues with
>> that approach and why it should not be recommended.
> Yeah, that approach is one of the typical examples that you should
> not do.
>> There are three cases I can come up with that would make users want a
>> readonly disk.
>> 1, share a disk across many initiators as readonly and avoid having
>> the various initiators write and update atime when a file is accessed.
>> this is a weak argument since one could just mount the fs with noatime
> Yeah, and if file systems try to write to a read-only lu and get an
> error, then the majority of file systems just stop working. So the
> logic to setting a lu read-only behind a file system doesn't work.
>> 2, snapshots. many filesystems support fs or file level snapshots.
>> Many of them only support readonly access to the snapshot device/file.
>> making the lun signal it is readonly back to the initiator reduces the
>> amount of surpsrise to an initiator when it does a mode sense and the
>> device says read-write ok but when it later tries a write to update a
>> inode atime its getting a sense code back.
>> 3, maybe you just don trust some initiators and just dont want them to
>> be able to write to the device ?
> Well, then you need to configure 'read-only' attribute per initiator?
> It doesn't look the patch supports it?
No the patch is per target so to have some initiators have read-write
and others have read-only would require two separate targets.
One target read-only and one read-write.Both targets exporting the
same physical file as a lun.
> I want to wait until someone shows the greatly useful stories of this
> feature. My NetApp box doesn't support such feature, so I guess that
> we are fine without it, I guess.
That is fine.
Netapp however is mainly a nas box, where iscsi is primarily a
value-add for a cifs server so you can also host your exchange
database on this very expensive server. I dont think this is
comparable to requirements or usecases for dedicated san/iscsi boxens
and gateways like cisco96xx, symmetrix, lhn, falcon, and similar.
Windows until very very recently could just not handle read-only
storage disks at all, no matter what since ntfs would become very
unhappy. So for something aimed primarily at ntfs and windows,
readonly would be pointless.
I dont think this feature is that vital. I think it is useful but not
vital. We can add it later if there is big demand for it.
If you dotn want to add it, that is fine.
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