From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39746) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YOxd8-0002qN-LS for qemu-devel@nongnu.org; Fri, 20 Feb 2015 19:09:12 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YOxd3-0001Kj-KY for qemu-devel@nongnu.org; Fri, 20 Feb 2015 19:09:10 -0500 Received: from mx1.redhat.com ([209.132.183.28]:47334) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YOxd3-0001Ka-D2 for qemu-devel@nongnu.org; Fri, 20 Feb 2015 19:09:05 -0500 Message-ID: <54E7CC9F.2090308@redhat.com> Date: Fri, 20 Feb 2015 19:09:03 -0500 From: John Snow MIME-Version: 1.0 References: <1424473645-29161-1-git-send-email-jsnow@redhat.com> <1424473645-29161-20-git-send-email-jsnow@redhat.com> <54E7C984.2070305@redhat.com> In-Reply-To: <54E7C984.2070305@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v14 19/19] docs: incremental backup documentation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eric Blake , qemu-devel@nongnu.org Cc: kwolf@redhat.com, famz@redhat.com, armbru@redhat.com, mreitz@redhat.com, vsementsov@parallels.com, stefanha@redhat.com On 02/20/2015 06:55 PM, Eric Blake wrote: > On 02/20/2015 04:07 PM, John Snow wrote: >> Signed-off-by: John Snow >> --- >> docs/bitmaps.md | 253 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >> 1 file changed, 253 insertions(+) >> create mode 100644 docs/bitmaps.md >> >> diff --git a/docs/bitmaps.md b/docs/bitmaps.md >> new file mode 100644 >> index 0000000..7cda146 >> --- /dev/null >> +++ b/docs/bitmaps.md >> @@ -0,0 +1,253 @@ >> +# Dirty Bitmaps > > No copyright/license? Might be good to add. > OK. > Also, I'm a fan of reviewing docs before code (does the design make > sense in isolation, and then did we implement it) rather than last in > the series (we may have faithfully documented our code, but that > includes baking in any design flaws that reviewers are now blind to > because of reading the code) > Sorry, I definitely did write this *after* and I selfishly put it at the end of the series to avoid disrupting the patch numbers from the previous revision. >> +* To create a new bitmap that tracks changes in 32KiB segments: >> + >> +```json >> +{ "execute": "block-dirty-bitmap-add", >> + "arguments": { >> + "node": "drive0", >> + "name": "bitmap0", >> + "granularity": "32768" > > s/"32768"/32768/ (we are using a JSON integer, not string) > >> + } >> +} >> +``` >> + >> +### Deletion >> + >> +* Can be performed on a disabled bitmap, but not a frozen one. >> + >> +* Because bitmaps are only unique to the node to which they are attached, >> +you must specify the node/drive name here, too. >> + >> +```json >> +{ "execute": "block-dirty-bitmap-remove", >> + "arguments": { >> + "node": "drive0", >> + "name": "bitmap0", >> + } > > No trailing commas in JSON. > >> +} >> +``` >> + >> +### Enable/Disable: >> + >> +* Not very useful in current cases, but potentially useful for debugging in the >> +future where we'd like to see what information changed only in a specific >> +time period: >> + >> +* To enable (which is, again, the default state after add) >> + >> +```json >> +{ "execute": "block-dirty-bitmap-enable", >> + "arguments": { >> + "node": "drive0", >> + "name": "bitmap0", >> + } > > and again. > >> +} >> +``` >> + >> +* To disable: >> + >> +```json >> +{ "execute": "block-dirty-bitmap-disable", >> + "arguments": { >> + "node": "drive0", >> + "name": "bitmap0", >> + } >> +} >> +``` > > and again. Also, maybe swap these two, since a bitmap defaults to > enabled (that is, you would logically use disable first, then enable to > re-enable, when testing these out, if there is no way to create an > already-disabled bitmap). > >> + >> +### Resetting >> + >> +* Resetting a bitmap will clear all information it holds. >> +* An incremental backup created from an empty bitmap will copy no data, >> +as if nothing has changed. >> + >> +```json >> +{ "execute": "block-dirty-bitmap-clear", >> + "arguments": { >> + "node": "drive0", >> + "name": "bitmap0", >> + } > > I'll quit pointing out broken trailing JSON commas, on the assumption > that you'll fix all of them rather than stopping here :) > Yeah, sorry! I have discovered today that I am *awful* at writing out json by hand. I promise the commands work otherwise! I will fix up the documentation here; if this winds up being the *ONLY* thing wrong with V14, I would prefer this doc be dropped from this series and I will just send a v2 for the doc by itself afterwards. >> +} >> +``` >> + >> +## Transactions >> + >> +### Justification >> +Bitmaps can be safely modified when the VM is paused or halted by using >> +the basic QMP commands. For instance, you might perform the following actions: >> + >> +1. Boot the VM in a paused state. >> +2. Create a full drive backup of drive0 >> +3. Create a new bitmap attached to drive0 >> +4. resume execution of the VM >> +5. Incremental backups are ready to be created. > > Consistency on using trailing '.'? > >> + >> +At this point, the bitmap and drive backup would be correctly in sync, >> +and incremental backups made from this point forward would be correctly aligned >> +to the full drive backup. >> + >> +This is not particularly useful if we decide we want to start incremental >> +backups after the VM has been running for a while, which will allow us to >> +perform actions like the following: >> + >> +1. Boot the VM and begin execution >> +2. Using transactions, perform the following operations: >> + * Create 'bitmap0' >> + * Create a full drive backup of drive0 >> +3. Incremental backups are now ready to be created. > > It's unclear whether point 2 is describing two separate transactions, or > one transaction with two steps. If it's just one, I'd word it: > > Using a single transaction, perform the following operations: > > OK. >> + >> +The star of the show. >> + >> +**Nota Bene!** Only incremental backups of entire drives are supported for now. >> +So despite the fact that you can attach a bitmap to any arbitrary node, they are >> +only currently useful when attached to the root node. This is because >> +drive-backup only supports drives/devices instead of arbitrary nodes. > > Maybe it's time to think about adding a node-backup :) But doesn't have > to be this series. > > I will crawl before I walk. And certainly before I start running and flying :) Thanks, --JS