btrfs subvolume as an iSCSI target?

Chris Murphy lists at colorremedies.com
Thu Mar 13 00:37:22 UTC 2014


On Mar 12, 2014, at 6:18 PM, Rick Stevens <ricks at alldigital.com> wrote:

> On 03/12/2014 04:23 PM, Dan Mossor issued this missive:
>> I've been combing the web for a couple days, and most hits I'm finding
>> regarding using a btrfs subvolume as an iSCSI target are posts from the
>> 2009/2010 timeframe stating that that functionality is broken.
>> 
>> I'm working on a couple projects here, and one of those involves the
>> research into iSCSI targets and initiators, and the testing thereof. I
>> have an F20 system with a 1.3TB btrfs master subvolume (across three
>> dissimilar devices), and want to create a subvolume within that to serve
>> as an iSCSI target.
>> 
>> The traditional methods, specifically those mentioned in the Fedora Wiki
>> and the tgtadm man pages, place the emphasis on using devices (/dev/sdx)
>> or files (dd if=/dev/zero of=/array/target1 bs=1k count=1). I realize
>> that using the file is probably the easiest thing to do in this
>> instance, but that's not the point of my project - I am intentionally
>> trying to designate the btrfs subvolume as the target.
>> 
>> The issue I believe that is going to stop me is that tgtadm expects a
>> block device as the target, and btrfs does not provide that. Is there a
>> workaround, other than using a file in the subvolume as the target? Is
>> there work in progress to enable this feature, either in the
>> scsi-target-utils package or within the btrfs framework?
> 
> iSCSI stands for "internet SCSI", a way to share raw devices over normal
> network mechanisms rather than having to have a storage farm (e.g.
> fiberchannel switches or SAS concentrators and a ton of cables). As far
> as I know, the functionality you want will never be available. iSCSI
> is intended to share raw devices, not subdirectories (that's what things
> like NFS and CIFS are for).

It's a reasonable inquiry as ZFS does support iSCSI export of a zvol, but this is built into ZFS directly.

https://btrfs.wiki.kernel.org/index.php/Project_ideas#block_devices_.27btrvols.27

No one has taken this yet though, so no work so far.


Chris Murphy



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