Yes, it's quite possible that for simplicity sake the cache tier OSD
writes back the full object to the base tier even if only a single
byte has changed. In that case, while not optimal behavior, at least
if you combine the base + cache tier snapsets together, RBD should be
able to export enough data to reconstruct the image.
On Thu, Aug 10, 2017 at 3:06 AM, Xuehan Xu <xxhdx1985126@xxxxxxxxx> wrote:
> Hi, Jason.
>
> I did a test, it turned out that, after flushing the object out of the
> cache tier, the clone overlap in base tier changed to empty, as is
> shown below. I think this maybe because that the flush operation just
> mark the whole range of the object being flushed as modified, so if
> the object's size hasn't changed, the overlap becomes empty. Is this
> right?
>
> Thank you:-)
>
> {
> "id": {
> "oid": "test.obj",
> "key": "",
> "snapid": -2,
> "hash": 3575411564,
> "max": 0,
> "pool": 10,
> "namespace": "",
> "max": 0
> },
> "info": {
> "oid": {
> "oid": "test.obj",
> "key": "",
> "snapid": -2,
> "hash": 3575411564,
> "max": 0,
> "pool": 10,
> "namespace": ""
> },
> "version": "4876'9",
> "prior_version": "4854'8",
> "last_reqid": "osd.35.4869:1",
> "user_version": 16,
> "size": 4194303,
> "mtime": "2017-08-10 14:54:56.087387",
> "local_mtime": "2017-08-10 14:59:15.252755",
> "lost": 0,
> "flags": 52,
> "snaps": [],
> "truncate_seq": 0,
> "truncate_size": 0,
> "data_digest": 2827420887,
> "omap_digest": 4294967295,
> "watchers": {}
> },
> "stat": {
> "size": 4194303,
> "blksize": 4096,
> "blocks": 8200,
> "nlink": 1
> },
> "SnapSet": {
> "snap_context": {
> "seq": 3,
> "snaps": [
> 3
> ]
> },
> "head_exists": 1,
> "clones": [
> {
> "snap": 3,
> "size": 4194303,
> "overlap": "[]"
> }
> ]
> }
> }
>
> On 9 August 2017 at 23:26, Jason Dillaman <jdillama@xxxxxxxxxx> wrote:
>> If you flush the object out of the cache tier so that its changes are
>> recorded in the base tier, is the overlap correctly recorded in the
>> base tier?
>>
>> On Wed, Aug 9, 2017 at 12:24 AM, Xuehan Xu <xxhdx1985126@xxxxxxxxx> wrote:
>>> By the way, according to our test, since the modified range is not
>>> recorded either in the cache tier or in the base tier, I think
>>> proxying the "list-snaps" down to the base tier might not work as
>>> well, is that right?
>>>
>>> On 9 August 2017 at 12:20, Xuehan Xu <xxhdx1985126@xxxxxxxxx> wrote:
>>>> Sorry, I didn't "reply all"....:-)
>>>>
>>>>
>>>> ---------- Forwarded message ----------
>>>> From: Xuehan Xu <xxhdx1985126@xxxxxxxxx>
>>>> Date: 9 August 2017 at 12:14
>>>> Subject: Re: About the problem "export_diff relies on clone_overlap,
>>>> which is lost when cache tier is enabled"
>>>> To: dillaman@xxxxxxxxxx
>>>>
>>>>
>>>> Um, no, I don't think they are related.
>>>>
>>>> My problem is this:
>>>>
>>>> At first , we tried to use "rbd export-diff" to do incremental backup
>>>> on Jewel verion ceph cluster which is cache-tier enabled. However,
>>>> when we compare the original rbd image and the backup rbd image, we
>>>> find that they are different. After a series of debugging, we found
>>>> that this is because WRITE ops' "modified_range" is not substracted
>>>> from the clone overlap of its targeting object's HEAD version when
>>>> that object's HEAD verion is in cache iter and its most recent clone
>>>> object is not, which led to the miscalculation of the
>>>> "calc_snap_set_diff" function.
>>>>
>>>> For example, we did such a test: we first made created a snap for an
>>>> rbd image "test.2.img" whose size is only 4MB which means it only
>>>> contains one object; then, we sent a series of AioWrites to
>>>> "test.2.img" to promote its HEAD object into cache tier, while leaving
>>>> its clone object in the base tier only; at that time, we used
>>>> "ceph-objectstore-tool" to dump the object we wrote to, and the result
>>>> was as follows:
>>>>
>>>> {
>>>> "id": {
>>>> "oid": "rbd_data.2aae62ae8944a.0000000000000000",
>>>> "key": "",
>>>> "snapid": -2,
>>>> "hash": 2375431681,
>>>> "max": 0,
>>>> "pool": 4,
>>>> "namespace": "",
>>>> "max": 0
>>>> },
>>>> "info": {
>>>> "oid": {
>>>> "oid": "rbd_data.2aae62ae8944a.0000000000000000",
>>>> "key": "",
>>>> "snapid": -2,
>>>> "hash": 2375431681,
>>>> "max": 0,
>>>> "pool": 4,
>>>> "namespace": ""
>>>> },
>>>> "version": "4536'2728",
>>>> "prior_version": "4536'2727",
>>>> "last_reqid": "client.174858.0:10",
>>>> "user_version": 14706,
>>>> "size": 68,
>>>> "mtime": "2017-08-09 11:30:18.263983",
>>>> "local_mtime": "2017-08-09 11:30:18.264310",
>>>> "lost": 0,
>>>> "flags": 4,
>>>> "snaps": [],
>>>> "truncate_seq": 0,
>>>> "truncate_size": 0,
>>>> "data_digest": 4294967295,
>>>> "omap_digest": 4294967295,
>>>> "watchers": {}
>>>> },
>>>> "stat": {
>>>> "size": 68,
>>>> "blksize": 4096,
>>>> "blocks": 16,
>>>> "nlink": 1
>>>> },
>>>> "SnapSet": {
>>>> "snap_context": {
>>>> "seq": 28,
>>>> "snaps": [
>>>> 28
>>>> ]
>>>> },
>>>> "head_exists": 1,
>>>> "clones": [
>>>> {
>>>> "snap": 28,
>>>> "size": 68,
>>>> "overlap": "[0~64]"
>>>> }
>>>> ]
>>>> }
>>>> }
>>>>
>>>> In this result, we found that the overlap for clone "28" is [0~64]. So
>>>> we specifically sent a AioWrite req to this object to write to the
>>>> offset 32 with 4 bytes of ramdon data, which we thought the overlap
>>>> should change to [0~32, 36~64]. However, the result is as follows:
>>>>
>>>> {
>>>> "id": {
>>>> "oid": "rbd_data.2aae62ae8944a.0000000000000000",
>>>> "key": "",
>>>> "snapid": -2,
>>>> "hash": 2375431681,
>>>> "max": 0,
>>>> "pool": 4,
>>>> "namespace": "",
>>>> "max": 0
>>>> },
>>>> "info": {
>>>> "oid": {
>>>> "oid": "rbd_data.2aae62ae8944a.0000000000000000",
>>>> "key": "",
>>>> "snapid": -2,
>>>> "hash": 2375431681,
>>>> "max": 0,
>>>> "pool": 4,
>>>> "namespace": ""
>>>> },
>>>> "version": "4546'2730",
>>>> "prior_version": "4538'2729",
>>>> "last_reqid": "client.155555.0:10",
>>>> "user_version": 14708,
>>>> "size": 4096,
>>>> "mtime": "2017-08-09 11:36:20.717910",
>>>> "local_mtime": "2017-08-09 11:36:20.719039",
>>>> "lost": 0,
>>>> "flags": 4,
>>>> "snaps": [],
>>>> "truncate_seq": 0,
>>>> "truncate_size": 0,
>>>> "data_digest": 4294967295,
>>>> "omap_digest": 4294967295,
>>>> "watchers": {}
>>>> },
>>>> "stat": {
>>>> "size": 4096,
>>>> "blksize": 4096,
>>>> "blocks": 16,
>>>> "nlink": 1
>>>> },
>>>> "SnapSet": {
>>>> "snap_context": {
>>>> "seq": 28,
>>>> "snaps": [
>>>> 28
>>>> ]
>>>> },
>>>> "head_exists": 1,
>>>> "clones": [
>>>> {
>>>> "snap": 28,
>>>> "size": 68,
>>>> "overlap": "[0~64]"
>>>> }
>>>> ]
>>>> }
>>>> }
>>>>
>>>> It's obvious that it didn't change at all. If we do export-diff under
>>>> this circumstance, the result would be wrong. Meanwhile, in the base
>>>> tier, the "ceph-objectstore-tool" dump's result also showed that the
>>>> overlap recorded in the base tier didn't change also:
>>>> {
>>>> "id": {
>>>> "oid": "rbd_data.2aae62ae8944a.0000000000000000",
>>>> "key": "",
>>>> "snapid": -2,
>>>> "hash": 2375431681,
>>>> "max": 0,
>>>> "pool": 3,
>>>> "namespace": "",
>>>> "max": 0
>>>> },
>>>> "info": {
>>>> "oid": {
>>>> "oid": "rbd_data.2aae62ae8944a.0000000000000000",
>>>> "key": "",
>>>> "snapid": -2,
>>>> "hash": 2375431681,
>>>> "max": 0,
>>>> "pool": 3,
>>>> "namespace": ""
>>>> },
>>>> "version": "4536'14459",
>>>> "prior_version": "4536'14458",
>>>> "last_reqid": "client.174834.0:10",
>>>> "user_version": 14648,
>>>> "size": 68,
>>>> "mtime": "2017-08-09 11:30:01.412634",
>>>> "local_mtime": "2017-08-09 11:30:01.413614",
>>>> "lost": 0,
>>>> "flags": 36,
>>>> "snaps": [],
>>>> "truncate_seq": 0,
>>>> "truncate_size": 0,
>>>> "data_digest": 4294967295,
>>>> "omap_digest": 4294967295,
>>>> "watchers": {}
>>>> },
>>>> "stat": {
>>>> "size": 68,
>>>> "blksize": 4096,
>>>> "blocks": 16,
>>>> "nlink": 1
>>>> },
>>>> "SnapSet": {
>>>> "snap_context": {
>>>> "seq": 28,
>>>> "snaps": [
>>>> 28
>>>> ]
>>>> },
>>>> "head_exists": 1,
>>>> "clones": [
>>>> {
>>>> "snap": 28,
>>>> "size": 68,
>>>> "overlap": "[0~64]"
>>>> }
>>>> ]
>>>> }
>>>> }
>>>>
>>>> Then we turn to the source code to find the reason for this. And we
>>>> found that, the reason should be that, in the
>>>> ReplicatedPG::make_writeable method, when determining whether the
>>>> write op's modified range should be subtracted from the clone overlap,
>>>> it has pass two condition check: "ctx->new_snapset.clones.size() > 0"
>>>> and "is_present_clone(last_clone_oid)", as the code below shows.
>>>>
>>>> // update most recent clone_overlap and usage stats
>>>> if (ctx->new_snapset.clones.size() > 0) {
>>>> /* we need to check whether the most recent clone exists, if it's
>>>> been evicted,
>>>> * it's not included in the stats */
>>>> hobject_t last_clone_oid = soid;
>>>> last_clone_oid.snap = ctx->new_snapset.clone_overlap.rbegin()->first;
>>>> if (is_present_clone(last_clone_oid)) {
>>>> interval_set<uint64_t> &newest_overlap =
>>>> ctx->new_snapset.clone_overlap.rbegin()->second;
>>>> ctx->modified_ranges.intersection_of(newest_overlap);
>>>> // modified_ranges is still in use by the clone
>>>> add_interval_usage(ctx->modified_ranges, ctx->delta_stats);
>>>> newest_overlap.subtract(ctx->modified_ranges);
>>>> }
>>>> }
>>>>
>>>> We thought that the latter condition check
>>>> "is_present_clone(last_clone_oid)" might not be reasonable to be a
>>>> judgement base for the determination of whether to subtract the clone
>>>> overlap with write ops' modified range, so we changed to code above to
>>>> the following, which moved the subtraction out of the latter condition
>>>> check, and submitted a pr for this:
>>>> https://github.com/ceph/ceph/pull/16790:
>>>>
>>>> // update most recent clone_overlap and usage stats
>>>> if (ctx->new_snapset.clones.size() > 0) {
>>>> /* we need to check whether the most recent clone exists, if it's
>>>> been evicted,
>>>> * it's not included in the stats */
>>>> hobject_t last_clone_oid = soid;
>>>> last_clone_oid.snap = ctx->new_snapset.clone_overlap.rbegin()->first;
>>>> interval_set<uint64_t> &newest_overlap =
>>>> ctx->new_snapset.clone_overlap.rbegin()->second;
>>>> ctx->modified_ranges.intersection_of(newest_overlap);
>>>> newest_overlap.subtract(ctx->modified_ranges);
>>>>
>>>> if (is_present_clone(last_clone_oid)) {
>>>> // modified_ranges is still in use by the clone
>>>> add_interval_usage(ctx->modified_ranges, ctx->delta_stats);
>>>> }
>>>> }
>>>>
>>>> In our test, this change solved the problem successfully, however, we
>>>> can't confirm that this change won't cause any new problems. So, here
>>>> we are discussing how to solve this problem:-)
>>>>
>>>> On 9 August 2017 at 09:26, Jason Dillaman <jdillama@xxxxxxxxxx> wrote:
>>>>> Is this issue related to https://github.com/ceph/ceph/pull/10626?
>>>>>
>>>>> On Mon, Aug 7, 2017 at 8:07 PM, Xuehan Xu <xxhdx1985126@xxxxxxxxx> wrote:
>>>>>> OK, I'll try that. Thank you:-)
>>>>>>
>>>>>> On 8 August 2017 at 04:48, Jason Dillaman <jdillama@xxxxxxxxxx> wrote:
>>>>>>> Could you just proxy the "list snaps" op from the cache tier down to
>>>>>>> the base tier and combine the cache tier + base tier results? Reading
>>>>>>> the associated ticket, it seems kludgy to me to attempt to work around
>>>>>>> this within librbd by just refusing the provide intra-object diffs if
>>>>>>> cache tiering is in-use.
>>>>>>>
>>>>>>> On Sat, Aug 5, 2017 at 11:56 AM, Xuehan Xu <xxhdx1985126@xxxxxxxxx> wrote:
>>>>>>>> Hi, everyone.
>>>>>>>>
>>>>>>>> Trying to solve the issue "http://tracker.ceph.com/issues/20896";, I
>>>>>>>> just did another test: I did some writes to an object
>>>>>>>> "rbd_data.1ebc6238e1f29.0000000000000000" to raise its "HEAD" object
>>>>>>>> to the cache tier, after which I specifically write to its offset 0x40
>>>>>>>> with 4 bytes of random data. Then I used "ceph-objectstore-tool" to
>>>>>>>> dump its "HEAD" version in the base tier, the result is as
>>>>>>>> follows(before I raise it to cache tier, there is three snaps the
>>>>>>>> latest of which is 26):
>>>>>>>>
>>>>>>>> {
>>>>>>>> "id": {
>>>>>>>> "oid": "rbd_data.1ebc6238e1f29.0000000000000000",
>>>>>>>> "key": "",
>>>>>>>> "snapid": -2,
>>>>>>>> "hash": 1655893237,
>>>>>>>> "max": 0,
>>>>>>>> "pool": 3,
>>>>>>>> "namespace": "",
>>>>>>>> "max": 0
>>>>>>>> },
>>>>>>>> "info": {
>>>>>>>> "oid": {
>>>>>>>> "oid": "rbd_data.1ebc6238e1f29.0000000000000000",
>>>>>>>> "key": "",
>>>>>>>> "snapid": -2,
>>>>>>>> "hash": 1655893237,
>>>>>>>> "max": 0,
>>>>>>>> "pool": 3,
>>>>>>>> "namespace": ""
>>>>>>>> },
>>>>>>>> "version": "4219'16423",
>>>>>>>> "prior_version": "3978'16310",
>>>>>>>> "last_reqid": "osd.70.4213:2359",
>>>>>>>> "user_version": 17205,
>>>>>>>> "size": 4194304,
>>>>>>>> "mtime": "2017-08-03 22:07:34.656122",
>>>>>>>> "local_mtime": "2017-08-05 23:02:33.628734",
>>>>>>>> "lost": 0,
>>>>>>>> "flags": 52,
>>>>>>>> "snaps": [],
>>>>>>>> "truncate_seq": 0,
>>>>>>>> "truncate_size": 0,
>>>>>>>> "data_digest": 2822203961,
>>>>>>>> "omap_digest": 4294967295,
>>>>>>>> "watchers": {}
>>>>>>>> },
>>>>>>>> "stat": {
>>>>>>>> "size": 4194304,
>>>>>>>> "blksize": 4096,
>>>>>>>> "blocks": 8200,
>>>>>>>> "nlink": 1
>>>>>>>> },
>>>>>>>> "SnapSet": {
>>>>>>>> "snap_context": {
>>>>>>>> "seq": 26,
>>>>>>>> "snaps": [
>>>>>>>> 26,
>>>>>>>> 25,
>>>>>>>> 16
>>>>>>>> ]
>>>>>>>> },
>>>>>>>> "head_exists": 1,
>>>>>>>> "clones": [
>>>>>>>> {
>>>>>>>> "snap": 16,
>>>>>>>> "size": 4194304,
>>>>>>>> "overlap": "[4~4194300]"
>>>>>>>> },
>>>>>>>> {
>>>>>>>> "snap": 25,
>>>>>>>> "size": 4194304,
>>>>>>>> "overlap": "[]"
>>>>>>>> },
>>>>>>>> {
>>>>>>>> "snap": 26,
>>>>>>>> "size": 4194304,
>>>>>>>> "overlap": "[]"
>>>>>>>> }
>>>>>>>> ]
>>>>>>>> }
>>>>>>>> }
>>>>>>>>
>>>>>>>> As we can see, its clone_overlap for snap 26 is empty, which,
>>>>>>>> combining with the previous test described in
>>>>>>>> http://tracker.ceph.com/issues/20896, means that the writes' "modified
>>>>>>>> range" is neither recorded in the cache tier nor in the base tier.
>>>>>>>>
>>>>>>>> I think maybe we really should move the clone overlap modification out
>>>>>>>> of the IF block which has the condition check "is_present_clone". As
>>>>>>>> for now, I can't see any other way to fix this problem.
>>>>>>>>
>>>>>>>> Am I right about this?
>>>>>>>>
>>>>>>>> On 4 August 2017 at 03:14, Xuehan Xu <xxhdx1985126@xxxxxxxxx> wrote:
>>>>>>>>> I mean I think it's the condition check "is_present_clone" that
>>>>>>>>> prevent the clone overlap to record the client write operations
>>>>>>>>> modified range when the target "HEAD" object exists without its most
>>>>>>>>> recent clone object, and if I'm right, just move the clone overlap
>>>>>>>>> modification out of the "is_present_clone" condition check block is
>>>>>>>>> enough to solve this case, just like the PR
>>>>>>>>> "https://github.com/ceph/ceph/pull/16790";, and this fix wouldn't cause
>>>>>>>>> other problems.
>>>>>>>>>
>>>>>>>>> In our test, this fix solved the problem successfully, however, we
>>>>>>>>> can't confirm it won't cause new problems yet.
>>>>>>>>>
>>>>>>>>> So if anyone see this and knows the answer, please help us. Thank you:-)
>>>>>>>>>
>>>>>>>>> On 4 August 2017 at 11:41, Xuehan Xu <xxhdx1985126@xxxxxxxxx> wrote:
>>>>>>>>>> Hi, grep:-)
>>>>>>>>>>
>>>>>>>>>> I finally got what you mean in https://github.com/ceph/ceph/pull/16790.
>>>>>>>>>>
>>>>>>>>>> I agree with you in that " clone overlap is supposed to be tracking
>>>>>>>>>> which data is the same on disk".
>>>>>>>>>>
>>>>>>>>>> My thought is that, "ObjectContext::new_snapset.clones" is already an
>>>>>>>>>> indicator about whether there are clone objects on disk, so, in the
>>>>>>>>>> scenario of "cache tier", although a clone oid does not corresponds to
>>>>>>>>>> a "present clone" in cache tier, as long as
>>>>>>>>>> "ObjectContext::new_snapset.clones" is not empty, there must a one
>>>>>>>>>> such clone object in the base tier. And, as long as
>>>>>>>>>> "ObjectContext::new_snapset.clones" has a strict "one-to-one"
>>>>>>>>>> correspondence to "ObjectContext::new_snapset.clone_overlap", passing
>>>>>>>>>> the condition check "if (ctx->new_snapset.clones.size() > 0)" is
>>>>>>>>>> enough to make the judgement that the clone object exists.
>>>>>>>>>>
>>>>>>>>>> So, if I'm right, passing the condition check "if
>>>>>>>>>> (ctx->new_snapset.clones.size() > 0)" is already enough for us to do
>>>>>>>>>> "newest_overlap.subtract(ctx->modified_ranges)", it doesn't have to
>>>>>>>>>> pass "is_present_clone".
>>>>>>>>>>
>>>>>>>>>> Am I right about this? Or am I missing anything?
>>>>>>>>>>
>>>>>>>>>> Please help us, thank you:-)
>>>>>>>> --
>>>>>>>> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
>>>>>>>> the body of a message to majordomo@xxxxxxxxxxxxxxx
>>>>>>>> More majordomo info at http://vger.kernel.org/majordomo-info.html
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> Jason
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Jason
>>
>>
>>
>> --
>> Jason
--
Jason
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