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From: Mikulas Patocka <>
To: Paul Lawrence <>
Cc: Alasdair Kergon <>,
	Mike Snitzer <>,,,
	Jonathan Corbet <>,,,, Shaohua Li <>
Subject: Re: [dm-devel] [RFC] dm-bow working prototype
Date: Fri, 26 Oct 2018 16:03:10 -0400 (EDT)	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Thu, 25 Oct 2018, Paul Lawrence wrote:

> Thank you for the suggestion. I spent part of yesterday experimenting with
> this idea, and it is certainly very promising. However, it does have some
> disadvantages as compared to dm-bow, if I am understanding the setup
> correctly:
> 1) Since dm-snap has no concept of the free space on the underlying file
> system any write into the free space will trigger a backup, so using twice the
> space of dm-bow. Changing existing data will create a backup with both
> drivers, but since we have to reserve the space for the backups up-front with
> dm-snap, we would likely only have half the space for that. Either way, it
> seems that dm-bow is likely to double the amount of changes we could make.
> (Might it be possible to dynamically resize the backup file if it is mostly
> used up? This would fix the problem of only having half the space for changing

Yes - the snapshot store can be extended while the snapshot is active.

> existing data. The documentation seems to indicate that you can increase the
> size of the snapshot partition, and it seems like it should be possible to
> grow the underlying file without triggering a lot of writes. OTOH this would
> have to happen in userspace which creates other issues.)
> 2) Similarly, since writes into free space do not trigger a backup in dm-bow,
> dm-bow is likely to have a lower performance overhead in many circumstances.
> On the flip side, dm-bow's backup is in free space and will collide with other
> writes, so this advantage will reduce as free space fills up. But by choosing
> a suitable algorithm for how we use free space we might be able to retain most
> of this advantage.
> I intend to put together a fully working prototype of your suggestion next to
> better compare with dm-bow. But I do believe there is value in tracking free
> space and utilizing it in any such solution.

The snapshot target could be hacked so that it remembers space trimmed 
with REQ_OP_DISCARD and won't reallocate these blocks.

But I suspect that running discard over the whole device would degrade 
performance more than copying some unneeded data.

How much data do you intend to backup with this solution?


  reply	other threads:[~2018-10-26 20:03 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-23 21:23 [RFC] dm-bow working prototype Paul Lawrence
2018-10-23 22:18 ` Alasdair G Kergon
2018-10-24 18:42   ` Paul Lawrence
2018-10-24 19:24     ` [dm-devel] " Mikulas Patocka
2018-10-25  0:01       ` Alasdair G Kergon
2018-10-25 10:20       ` Bryn M. Reeves
2018-10-25 17:23       ` Paul Lawrence
2018-10-26 20:03         ` Mikulas Patocka [this message]
2018-10-29 16:51           ` Paul Lawrence
2018-11-15 23:15             ` Mikulas Patocka
2018-12-02 10:07               ` Sandeep Patil
2018-10-25 16:30 ` MegaBrutal
2018-10-25 18:13   ` Paul Lawrence
2018-10-25 21:15     ` Wols Lists
2018-10-25 21:43 ` [dm-devel] " Darrick J. Wong

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