Re: [Scst-devel] Thin Provisioning and Ceph RBD's

[Nick F. <ni...@fi...>]

> -----Original Message-----
> From: Alex Gorbachev [mailto:ag...@is...]
> Sent: 28 June 2016 08:39
> To: Nick Fisk <ni...@fi...>
> Cc: scs...@li...; Vladislav Bolkhovitin <vs...@vl...>
> Subject: Re: [Scst-devel] Thin Provisioning and Ceph RBD's
> 
> Hi Nick, Vlad,
> 
> On Tue, Jun 21, 2016 at 11:34 AM, Nick Fisk <ni...@fi...> wrote:
> >> Nick Fisk wrote on 06/20/2016 05:25 AM:
> >> > Hi,
> >> >
> >> > I'm trying to export some Ceph RBD's as iSCSI devices which is
> >> > working really well so far, but I cannot get SCST to pick them up
> >> > as thin provisioned. I'm using the fileIO driver in SCST to map
> >> > directly to the
> >> > /dev/rbd0 block device.
> ...
> 
> >> Yes, by using FILEIO you are treating your /dev/rbd0 entry as file.
> >> This means that for thin provisioning support it must provide fallocate
> support.
> >
> > Right understood, makes sense now.
> >
> >>
> >> > I can see just above this, that the code is checking if the block
> >> > device supports discard, so is this 2nd check needed when mapping
> >> > directly to a block device?
> >>
> >> If you want to rely on the discard support, you need to use BLOCKIO
> >> to treat
> >> /dev/rbd0 as a block device.
> >
> > So effectively for me it’s a choice between readahead or thin
> > provisioning....hmmm
> 
> Would ESXi thin provisioning option for a datastore help here at all?
> Or does all functionality of discard disappear if using a FILEIO with RBD?
> 
> I am about to run series of tests using VAAI, and would greatly appreciate
> any pointers to more info, or your thoughts.

SCST disables the thin flag and so ESXi doesn't see it as a thin disk, so no UNMAP commands can be sent. The main reason for me to use FILEIO over BLOCKIO was readahead + pagecache performance gains. 

I have been looking at using the rbd-nbd tool, so that the caching is provided by librbd and then use BLOCKIO with SCST. This will however need some work on the SCST resource agents to ensure the librbd cache is invalidated on ALUA state change.

The other thing I have seen is this

https://lwn.net/Articles/691871/

Which may mean FILEIO will support thin provisioning sometime in the future???

Nick

> 
> Regards,
> Alex
> 
> >
> >>
> >> Vlad
> >>
> >
> >
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