Re: [Qemu-devel] [PATCH v6 00/10] block: Incremental backup series

[John Snow]

On 11/18/2014 08:09 AM, Vladimir Sementsov-Ogievskiy wrote:
> > (3) Data Integrity
> >
> > The dirty flag could work something like:
> >
> > - If, on first open, the file has the dirty flag set, we need to
> > discard the bitmap data because we can no longer trust it.
> > - If the bitmap file is clean, proceed as normal, but take a lock
> > against any of the bitmap functions to prevent them from marking any
> > bits dirty.
> > - On first write to a clean persistent bitmap, delay the write until
> > we can mark the bitmap as dirty first. This incurs a write penalty
> > when we try to use the bitmap at first...
> > - Unlock the bitmap functions and allow them to mark blocks as needed.
> > - At some point, based on a sync policy, re-commit the dirty
> > information to the file and mark the file as clean once more and
> > re-take the persistence lock.
> Correct me if I'm wrong.
>
> #Read bitmap:
> read in blockdev_init, before any write to device, so no lock is needed.
>
> #Set bits in bitmap:
> if bitmap.dirty_flag:
>     set bits
> else:
>     LOCK
>     set bits
>     set bitmap.dirty_flag
>     set dirty_flag in bitmap file
>     UNLOCK
>
> #Sync:
> if not bitmap.dirty_flag:
>     skip sync
> else:
>     LOCK
>     save one of bitmap levels (saving the last one is too long and not
> very good idea, because it is fast-updateing)
>     unset dirty_flag in bitmap file
>     unset bitmap.dirty_flag
>     UNLOCK
>
> #Last sync in bdrv_close:
> Just save the last bitmap level and unset dirty_flag in bitmap file
>
> Also.. I'm not quite sure about locking.. As I understand, co-routines
> in qemu are not running in parallel, is locking required? Or sync timer
> will not be co-routine based?
>
> Best regards,
> Vladimir

Might be being too informal. I just meant a lock or barrier to prevent 
actual IO throughput until we can confirm the dirty flag has been 
adjusted to indicate that the persistent bitmap is now officially out of 
date. Nothing fancy.

Wasn't trying to imply that we needed threading protection, just 
"locking" the IO until we can configure the bitmap as we need it to be.

> On 18.11.2014 13:54, Vladimir Sementsov-Ogievskiy wrote:
> > > (2) File Format
> > >
> > > Some standard file magic, which includes:
> > >
> > > - Some magic byte(s)
> > > - Dirty flag. Needed to tell if we can trust this data or not.
> > > - The size of the bitmap
> > > - The granularity of the bitmap
> > > - The offset to the first sector of bitmap data (Maybe? It can't hurt
> > > if we give ourselves a sector's worth to write metadata within.)
> > > - Data starting at... PAGESIZE?
> > - The name of the bitmap and also the size of this name
> >
> > > (5) Partial Persistence
> > >
> > > We did not discuss only saving higher levels of the bitmap. What's
> > > the primary benefit you're seeking?
> > Hmm. It may be used for faster sync. Maybe, save some of bitmap levels
> > on timer while vm is running and save the last level on shutdown?
> >
> > CC qemu-devel - ok.
> >
> > Best regards,
> > Vladimir
> >
> > On 18.11.2014 02:46, John Snow wrote:
> > > On 11/13/2014 08:54 AM, Vladimir Sementsov-Ogievskiy wrote:
> > > > Hi
> > > >
> > > > I'd just like to start working on persistent dirty bitmap. My thoughts
> > > > about it are the following:
> > > > - qemu -drive file=file,dirty_bitmap=file
> > > >      so,  bitmap will be loaded with drive open and saved with drive
> > > > close.
> > > > - save only meaningful (the last) level of the bitmap, restore all
> > > > levels on bitmap loading
> > > > - bool parameter "persistent" for bdrv_create_dirty_bitmap and
> > > > BdrvDirtyBitmap
> > > > - internal dirty_bitmaps, saved in qcow2 file
> > > >
> > > > Best regards,
> > > > Vladimir
> > >
> > > I am thinking:
> > >
> > > (1) Command Lines
> > >
> > > If you enable dirty bitmaps and give it a file that doesn't exist, it
> > > should error out on you.
> > >
> > > If you enable dirty bitmaps and give it a file that's blank, it
> > > understands that it is to create a persistent bitmap file in this
> > > location and it should enable persistence.
> > >
> > > If a bitmap file is given and it has valid magic, this should imply
> > > persistence.
> > >
> > > I am hesitant to have it auto-create files that don't already exist
> > > in case the files become large in size and a misconfiguration leads
> > > to repeated creation of these files that get orphaned in random
> > > folders. Perhaps we can add a create=auto flag or similar to allow
> > > this behavior if wanted.
> > >
> > > (2) File Format
> > >
> > > Some standard file magic, which includes:
> > >
> > > - Some magic byte(s)
> > > - Dirty flag. Needed to tell if we can trust this data or not.
> > > - The size of the bitmap
> > > - The granularity of the bitmap
> > > - The offset to the first sector of bitmap data (Maybe? It can't hurt
> > > if we give ourselves a sector's worth to write metadata within.)
> > > - Data starting at... PAGESIZE?
> > >
> > > (3) Data Integrity
> > >
> > > The dirty flag could work something like:
> > >
> > > - If, on first open, the file has the dirty flag set, we need to
> > > discard the bitmap data because we can no longer trust it.
> > > - If the bitmap file is clean, proceed as normal, but take a lock
> > > against any of the bitmap functions to prevent them from marking any
> > > bits dirty.
> > > - On first write to a clean persistent bitmap, delay the write until
> > > we can mark the bitmap as dirty first. This incurs a write penalty
> > > when we try to use the bitmap at first...
> > > - Unlock the bitmap functions and allow them to mark blocks as needed.
> > > - At some point, based on a sync policy, re-commit the dirty
> > > information to the file and mark the file as clean once more and
> > > re-take the persistence lock.
> > >
> > > (4) Synchronization Policy
> > >
> > > - Sync after so many bits become dirty in the bitmap, either as an
> > > absolute threshold or a density percentage?
> > > - Sync periodically on a fixed timer?
> > > - Sync periodically opportunistically when I/O utilization becomes
> > > relatively low? (With some sort of starvation prevention timer?)
> > > - Sync only at shutdown?
> > >
> > > In discussing with Stefan, I think we rather liked the idea of a
> > > timer that tries to re-commit the block data during lulls in the I/O.
> > >
> > > (5) Partial Persistence
> > >
> > > We did not discuss only saving higher levels of the bitmap. What's
> > > the primary benefit you're seeking?
> > >
> > > (6) Inclusion as qcow2 Metadata
> > >
> > > And lastly, we did discuss the inclusion of the bitmap as qcow2
> > > metadata, but decided it wasn't our principle target for the format
> > > to allow generality to other file formats. We didn't really discuss
> > > the idea of having it as an option or an extension, but I don't (off
> > > the top of my head) have any reasonings against it, but I will likely
> > > not work on it myself.
> > >
> > >
> > > You didn't CC qemu-devel on this (so I won't!), but perhaps we should
> > > re-send out our ideas to the wider list for feedback before we
> > > proceed any further. Maybe we can split the work if we agree upon a
> > > design.
> > >
> > > Thanks!
> > > --js
> > >
> > > P.S.: I'm still cleaning up Fam's first patchset based on Max's and
> > > your feedback. Hope to have it out by the end of this week.
> > >
> > > > On 11.11.2014 18:59, John Snow wrote:
> > > > > On 11/10/2014 03:15 AM, Vladimir Sementsov-Ogievskiy wrote:
> > > > > > Hi Fam, hi Jorn.
> > > > > >
> > > > > > Jagane's project - http://wiki.qemu.org/Features/Livebackup
> > > > > >
> > > > > > In two words:
> > > > > > Normal delta - like in qemu, while backuping, we save all new
> > > > > > writes to
> > > > > > separate virtual disk - delta. When backup is done, we can merge
> > > > > > delta
> > > > > > back to original image.
> > > > > > Reverse delta - while backuping, we don't stop writing to original
> > > > > > image
> > > > > > (and qemu works with it, not with delta), but before every write
> > > > > > we copy
> > > > > > the corresponding block (if backup needs it) to separate virtual disk
> > > > > > (reverse delta). So, for backuping, if the current block is not
> > > > > > rewritten, we take it from original file, otherwise we take it from
> > > > > > reverse delta. The benefit is that, theoretically we'll have two
> > > > > > times
> > > > > > less overhead I/Os then with "normal delta backup + merge",
> > > > > > because of
> > > > > > part of (about 0.5) blocks which are rewritten while backuping
> > > > > > will be
> > > > > > backuped before rewrite, and they will not go to delta then. Also,
> > > > > > there
> > > > > > are should be methods to improve this coefficient, by choosing rules
> > > > > > about what blocks to backup first.
> > > > > >
> > > > > > Are someone work now on persistent (while vm is turned off)
> > > > > > bitmaps? If
> > > > > > not, are there any recommendations?
> > > > > >
> > > > > > Best regards,
> > > > > > Vladimir
> > > > >
> > > > > Hi Vladimir:
> > > > >
> > > > > I recently inherited Fam's backup series and I intended to start
> > > > > reviewing the comments this week. After I sift through that situation,
> > > > > the next step for me is persistent dirty bitmap support. I had been
> > > > > working on AHCI and IDE an awful lot lately, so it will take me a
> > > > > little bit to switch over.
> > > > >
> > > > > I don't have any existing notions of how I intend to implement it, so
> > > > > if you'd like to discuss any concerns you have about that subsystem,
> > > > > we can talk :)
> > > > >
> > > > > Thanks,
> > > > > --John
> > > > >
> > > > > > On 08.11.2014 10:19, Fam Zheng wrote:
> > > > > > > On Fri, 11/07 15:23, Vladimir Sementsov-Ogievskiy wrote:
> > > > > > > > Hi!
> > > > > > > >
> > > > > > > > Glad to see you working on backup series again. Some time ago I've
> > > > > > > > already
> > > > > > > > fixed v4 to be mergeable, but unfortunately didn't publish.
> > > > > > > > Actually I need (and I'm going to develop) incremental backup with
> > > > > > > > reverse
> > > > > > > > delta (like abandoned project
> > > > > > > > http://wiki.qemu.org/Features/Livebackup by
> > > > > > > > Jagane). And I've already fixed Jagane's project to be working with
> > > > > > > > current
> > > > > > > > Qemu. But the code is bad. Can you give me a peace of advice about
> > > > > > > > what
> > > > > > > > should I notice? I have a very superficial understanding of qemu
> > > > > > > > block
> > > > > > > > devices, block jobs, etc.
> > > > > > > Hi Vladimir,
> > > > > > >
> > > > > > > Thank you for reviewing my series.
> > > > > > >
> > > > > > > I haven't seen Jagane's series, so could you pleased explain what
> > > > > > > "reverse
> > > > > > > delta" is?
> > > > > > >
> > > > > > > John Snow (CC'ed) will take my work on incremental backup so I think
> > > > > > > you can
> > > > > > > coordinate. I'll probably count on John to work on your comments
> > > > > > > too,
> > > > > > > in the
> > > > > > > future I'll follow the progress and review.
> > > > > > >
> > > > > > > Thanks,
> > > > > > > Fam

--
—js