Re: [Cluster-devel] [Lsf-pc] [LSF/MM ATTEND] [TOPIC] fs/block interface discussions
Hi, On 11/12/14 00:52, Alasdair G Kergon wrote: On Wed, Dec 10, 2014 at 07:46:51PM +0100, Jan Kara wrote: But still you first need to stop all writes to the filesystem, then do a sync, and then allow writing again - which is exactly what freeze does. And with device-mapper, we were asked to support the taking of snapshots of multiple volumes simultaneously (e.g. where the application data is stored across more than one filesystem). Thin dm snapshots can handle this (the original non-thin ones can't). Alasdair Thats good to know, and a useful feature. One of the issues I can see is that because there are a number of different layers involved (application/fs/storage) coordination of requirements between those is not easy. To try to answer Jan's question earlier in the thread, no I don't know any specific application developers, but I can certainly help to propose some kind of solution, and then get some feedback. I think it is probably going to be easier to start with a specific proposal, albeit tentative, and then ask for feedback than to just say how should we do this? which is a lot more open ended. Going back to the other point above regarding freeze, is it not necessarily a requirement to stop all writes in order to do a snapshot, what is needed is in effect a barrier between operations which should be represented in the snapshot and those which should not, because they happen after the snapshot has been taken. Not that I'm particularly attached to that proposal as it stands, but I hope it demonstrates the kind of thing I had in mind for discussion. I hope also that it will be possible to come up with a better solution during and/or following the discussion. The goal would really be to figure out which bits we already have, which bits are missing, where the problems are, what can be done better, and so forth, while we have at least two of the three layers represented and in the same room. This is very much something for the long term rather than a quick discussion followed by a few patches kind of thing, I think, Steve.
Re: [Cluster-devel] [Lsf-pc] [LSF/MM ATTEND] [TOPIC] fs/block interface discussions
Hi, On Fri 12-12-14 11:46:34, Steven Whitehouse wrote: On 11/12/14 00:52, Alasdair G Kergon wrote: On Wed, Dec 10, 2014 at 07:46:51PM +0100, Jan Kara wrote: But still you first need to stop all writes to the filesystem, then do a sync, and then allow writing again - which is exactly what freeze does. And with device-mapper, we were asked to support the taking of snapshots of multiple volumes simultaneously (e.g. where the application data is stored across more than one filesystem). Thin dm snapshots can handle this (the original non-thin ones can't). Thats good to know, and a useful feature. One of the issues I can see is that because there are a number of different layers involved (application/fs/storage) coordination of requirements between those is not easy. To try to answer Jan's question earlier in the thread, no I don't know any specific application developers, but I can certainly help to propose some kind of solution, and then get some feedback. I think it is probably going to be easier to start with a specific proposal, albeit tentative, and then ask for feedback than to just say how should we do this? which is a lot more open ended. Going back to the other point above regarding freeze, is it not necessarily a requirement to stop all writes in order to do a snapshot, what is needed is in effect a barrier between operations which should be represented in the snapshot and those which should not, because they happen after the snapshot has been taken. Not that I'm particularly attached to that proposal as it stands, but I hope it demonstrates the kind of thing I had in mind for discussion. I hope also that it will be possible to come up with a better solution during and/or following the discussion. I think understand your idea with a 'barrier'. It's just that I have troubles seeing how it would actually get implemented - how do you make sure that e.g. after writing back block allocation bitmap and while writing back other metadata, noone can allocate new blocks to file 'foo' and still writeback the file's inode before you submit the barrier? The goal would really be to figure out which bits we already have, which bits are missing, where the problems are, what can be done better, and so forth, while we have at least two of the three layers represented and in the same room. This is very much something for the long term rather than a quick discussion followed by a few patches kind of thing, I think, Sure, if you have some proposal (not necessarily patches) then it's probably worth talking about. Honza -- Jan Kara j...@suse.cz SUSE Labs, CR
Re: [Cluster-devel] [Lsf-pc] [LSF/MM ATTEND] [TOPIC] fs/block interface discussions
Alex Elsayed wrote: Jan Kara wrote: Hi, On Fri 12-12-14 11:46:34, Steven Whitehouse wrote: On 11/12/14 00:52, Alasdair G Kergon wrote: On Wed, Dec 10, 2014 at 07:46:51PM +0100, Jan Kara wrote: But still you first need to stop all writes to the filesystem, then do a sync, and then allow writing again - which is exactly what freeze does. And with device-mapper, we were asked to support the taking of snapshots of multiple volumes simultaneously (e.g. where the application data is stored across more than one filesystem). Thin dm snapshots can handle this (the original non-thin ones can't). Thats good to know, and a useful feature. One of the issues I can see is that because there are a number of different layers involved (application/fs/storage) coordination of requirements between those is not easy. To try to answer Jan's question earlier in the thread, no I don't know any specific application developers, but I can certainly help to propose some kind of solution, and then get some feedback. I think it is probably going to be easier to start with a specific proposal, albeit tentative, and then ask for feedback than to just say how should we do this? which is a lot more open ended. Going back to the other point above regarding freeze, is it not necessarily a requirement to stop all writes in order to do a snapshot, what is needed is in effect a barrier between operations which should be represented in the snapshot and those which should not, because they happen after the snapshot has been taken. Not that I'm particularly attached to that proposal as it stands, but I hope it demonstrates the kind of thing I had in mind for discussion. I hope also that it will be possible to come up with a better solution during and/or following the discussion. I think understand your idea with a 'barrier'. It's just that I have troubles seeing how it would actually get implemented - how do you make sure that e.g. after writing back block allocation bitmap and while writing back other metadata, noone can allocate new blocks to file 'foo' and still writeback the file's inode before you submit the barrier? Actually, I suspect something could be (relatively) trivially implemented using a similar strategy to dm-era. Snapshots increment the era; blocks from previous eras cannot be overwritten or removed, and the target could be mapped to view a past era. With that, you have essentially instantaneous snapshots (increment a counter) with only a barrier constraint, not freezing. Thinking on it further, I'd suspect dm-thinp would also be fine with some form of barrier, rather than full freezing - generally speaking, if snapshots are (roughly) instantaneous, then we don't need to care so much about during the snapshot - ensuring a consistent state at one instant and preventing reordering across it should be sufficient. The goal would really be to figure out which bits we already have, which bits are missing, where the problems are, what can be done better, and so forth, while we have at least two of the three layers represented and in the same room. This is very much something for the long term rather than a quick discussion followed by a few patches kind of thing, I think, Sure, if you have some proposal (not necessarily patches) then it's probably worth talking about. Honza -- To unsubscribe from this list: send the line unsubscribe linux-fsdevel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [Cluster-devel] [Lsf-pc] [LSF/MM ATTEND] [TOPIC] fs/block interface discussions
Jan Kara wrote: Hi, On Fri 12-12-14 11:46:34, Steven Whitehouse wrote: On 11/12/14 00:52, Alasdair G Kergon wrote: On Wed, Dec 10, 2014 at 07:46:51PM +0100, Jan Kara wrote: But still you first need to stop all writes to the filesystem, then do a sync, and then allow writing again - which is exactly what freeze does. And with device-mapper, we were asked to support the taking of snapshots of multiple volumes simultaneously (e.g. where the application data is stored across more than one filesystem). Thin dm snapshots can handle this (the original non-thin ones can't). Thats good to know, and a useful feature. One of the issues I can see is that because there are a number of different layers involved (application/fs/storage) coordination of requirements between those is not easy. To try to answer Jan's question earlier in the thread, no I don't know any specific application developers, but I can certainly help to propose some kind of solution, and then get some feedback. I think it is probably going to be easier to start with a specific proposal, albeit tentative, and then ask for feedback than to just say how should we do this? which is a lot more open ended. Going back to the other point above regarding freeze, is it not necessarily a requirement to stop all writes in order to do a snapshot, what is needed is in effect a barrier between operations which should be represented in the snapshot and those which should not, because they happen after the snapshot has been taken. Not that I'm particularly attached to that proposal as it stands, but I hope it demonstrates the kind of thing I had in mind for discussion. I hope also that it will be possible to come up with a better solution during and/or following the discussion. I think understand your idea with a 'barrier'. It's just that I have troubles seeing how it would actually get implemented - how do you make sure that e.g. after writing back block allocation bitmap and while writing back other metadata, noone can allocate new blocks to file 'foo' and still writeback the file's inode before you submit the barrier? Actually, I suspect something could be (relatively) trivially implemented using a similar strategy to dm-era. Snapshots increment the era; blocks from previous eras cannot be overwritten or removed, and the target could be mapped to view a past era. With that, you have essentially instantaneous snapshots (increment a counter) with only a barrier constraint, not freezing. The goal would really be to figure out which bits we already have, which bits are missing, where the problems are, what can be done better, and so forth, while we have at least two of the three layers represented and in the same room. This is very much something for the long term rather than a quick discussion followed by a few patches kind of thing, I think, Sure, if you have some proposal (not necessarily patches) then it's probably worth talking about. Honza
Re: [Cluster-devel] [Lsf-pc] [LSF/MM ATTEND] [TOPIC] fs/block interface discussions
Hi, On Wed 10-12-14 11:49:48, Steven Whitehouse wrote: I'm interested generally in topics related to integration between components, one example being snapshots. We have snapshots at various different layers (can be done at array level or dm/lvm level and also we have filesystem support in the form of fs freezing). Well, usually snapshots at LVM layer are using fs freezing to get a consistent image of a filesystem. So these two are integrated AFAICS. There are a few thoughts that spring to mind - one being how this should integrate with applications - in order to make it easier to use, and another being whether we could introduce snapshots which do not require freezing the fs (as per btrfs) for other filesystems too - possibly by passing down a special kind of flush from the filesystem layer. So btrfs is special in its COW nature. For filesystems which do updates in place you can do COW in the block layer (after all that's what dm snapshotting does) but you still have to get fs into consistent state (that's fsfreeze), then take snapshot of the device (by setting up proper COW structures), and only then you can allow further modifications of the filesystem by unfreezing it. I don't see a way around that... A more general topic is proposed changes to the fs/block interface, of which the above may possibly be one example. There are a number of proposals for new classes of block device, and new features which will potentially require a different (or extended) interface at the fs/block layer. These have largely been discussed to date as individual features, and I wonder whether it might be useful to try and bring together the various proposals to see if there is commonality between at least some of them at the fs/block interface level. I know that there have been discussions going on relating to the individual proposals, so the idea I had was to try and look at them from a slightly different angle by bringing as many of them as possible together and concentrating on how they would be used from a filesystem perspective, Could you elaborate on which combination of features you'd like to discuss? Honza -- Jan Kara j...@suse.cz SUSE Labs, CR
Re: [Cluster-devel] [Lsf-pc] [LSF/MM ATTEND] [TOPIC] fs/block interface discussions
Hi, On 10/12/14 12:48, Jan Kara wrote: Hi, On Wed 10-12-14 11:49:48, Steven Whitehouse wrote: I'm interested generally in topics related to integration between components, one example being snapshots. We have snapshots at various different layers (can be done at array level or dm/lvm level and also we have filesystem support in the form of fs freezing). Well, usually snapshots at LVM layer are using fs freezing to get a consistent image of a filesystem. So these two are integrated AFAICS. There are a few thoughts that spring to mind - one being how this should integrate with applications - in order to make it easier to use, and another being whether we could introduce snapshots which do not require freezing the fs (as per btrfs) for other filesystems too - possibly by passing down a special kind of flush from the filesystem layer. So btrfs is special in its COW nature. For filesystems which do updates in place you can do COW in the block layer (after all that's what dm snapshotting does) but you still have to get fs into consistent state (that's fsfreeze), then take snapshot of the device (by setting up proper COW structures), and only then you can allow further modifications of the filesystem by unfreezing it. I don't see a way around that... Well I think it should be possible to get the fs into a consistent state without needing to do the freeze/snapshot/unfreeze procedure. Instead we might have (there are no doubt other solutions too, so this is just an example to get discussion started) an extra flag on a bio, which would only be valid with some combination of flush flags. Then it is just a case of telling the block layer that we want to do a snapshot, and it would then spot the marked bio when the fs sends it down, and know that everything before and including that bio should be in the snapshot, and everything after that is not. So the fs would do basically a special form of sync, setting the flag on the bio when it is consistent - the question being how should that then be triggered? It means that there is no longer any possibility of having a problem if the unfreeze does not happen for any reason. Perhaps the more important question though, is how it would/could be integrated with applications? The ultimate goal that I had in mind is that we could have a tool which is run to create a snapshot which will with a single command deal with all three (application/fs/block) layers, and it should not matter whether the snapshot is done via any particular fs or block device, it should work in the same way. So how could we send a message to a process to say that a snapshot is about to be taken, and to get a message back when the app has produced a consistent set of data, and to coordinate between multiple applications using the same block device, or even across multiple block devices, being used by a single app? A more general topic is proposed changes to the fs/block interface, of which the above may possibly be one example. There are a number of proposals for new classes of block device, and new features which will potentially require a different (or extended) interface at the fs/block layer. These have largely been discussed to date as individual features, and I wonder whether it might be useful to try and bring together the various proposals to see if there is commonality between at least some of them at the fs/block interface level. I know that there have been discussions going on relating to the individual proposals, so the idea I had was to try and look at them from a slightly different angle by bringing as many of them as possible together and concentrating on how they would be used from a filesystem perspective, Could you elaborate on which combination of features you'd like to discuss? Honza Well there are a number that I'm aware of that are currently in development, but I suspect that this list is not complete: - SMR drives - persistent memory (various different types) - Hinting from fs to block layer for various different reasons (layout, compression, snapshots, anything else?) - Better i/o error reporting/recovery - copy offload - anything I forgot? Steve.
Re: [Cluster-devel] [Lsf-pc] [LSF/MM ATTEND] [TOPIC] fs/block interface discussions
Hi, On Wed 10-12-14 14:13:02, Steven Whitehouse wrote: On 10/12/14 12:48, Jan Kara wrote: On Wed 10-12-14 11:49:48, Steven Whitehouse wrote: I'm interested generally in topics related to integration between components, one example being snapshots. We have snapshots at various different layers (can be done at array level or dm/lvm level and also we have filesystem support in the form of fs freezing). Well, usually snapshots at LVM layer are using fs freezing to get a consistent image of a filesystem. So these two are integrated AFAICS. There are a few thoughts that spring to mind - one being how this should integrate with applications - in order to make it easier to use, and another being whether we could introduce snapshots which do not require freezing the fs (as per btrfs) for other filesystems too - possibly by passing down a special kind of flush from the filesystem layer. So btrfs is special in its COW nature. For filesystems which do updates in place you can do COW in the block layer (after all that's what dm snapshotting does) but you still have to get fs into consistent state (that's fsfreeze), then take snapshot of the device (by setting up proper COW structures), and only then you can allow further modifications of the filesystem by unfreezing it. I don't see a way around that... Well I think it should be possible to get the fs into a consistent state without needing to do the freeze/snapshot/unfreeze procedure. Instead we might have (there are no doubt other solutions too, so this is just an example to get discussion started) an extra flag on a bio, which would only be valid with some combination of flush flags. Then it is just a case of telling the block layer that we want to do a snapshot, and it would then spot the marked bio when the fs sends it down, and know that everything before and including that bio should be in the snapshot, and everything after that is not. So the fs would do basically a special form of sync, setting the flag on the bio when it is consistent - the question being how should that then be triggered? It means that there is no longer any possibility of having a problem if the unfreeze does not happen for any reason. But still you first need to stop all writes to the filesystem, then do a sync, and then allow writing again - which is exactly what freeze does. Without stopping writers, you cannot be sure you don't have a mix of old and new files in the snapshot and also guranteeing some finite completion time is difficult (although that's doable)... So it seems to me that what you describe is freeze-snapshot-unfreeze cycle, just that it's fully controlled by the kernel. Perhaps the more important question though, is how it would/could be integrated with applications? The ultimate goal that I had in mind is that we could have a tool which is run to create a snapshot which will with a single command deal with all three (application/fs/block) layers, and it should not matter whether the snapshot is done via any particular fs or block device, it should work in the same way. So how could we send a message to a process to say that a snapshot is about to be taken, and to get a message back when the app has produced a consistent set of data, and to coordinate between multiple applications using the same block device, or even across multiple block devices, being used by a single app? Yeah, this would be nice. But it requires buy in from the applications which is always difficult. Do you know any application whose developers would be interested in something like this? A more general topic is proposed changes to the fs/block interface, of which the above may possibly be one example. There are a number of proposals for new classes of block device, and new features which will potentially require a different (or extended) interface at the fs/block layer. These have largely been discussed to date as individual features, and I wonder whether it might be useful to try and bring together the various proposals to see if there is commonality between at least some of them at the fs/block interface level. I know that there have been discussions going on relating to the individual proposals, so the idea I had was to try and look at them from a slightly different angle by bringing as many of them as possible together and concentrating on how they would be used from a filesystem perspective, Could you elaborate on which combination of features you'd like to discuss? Honza Well there are a number that I'm aware of that are currently in development, but I suspect that this list is not complete: - SMR drives - persistent memory (various different types) - Hinting from fs to block layer for various different reasons (layout, compression, snapshots, anything else?) - Better i/o error reporting/recovery - copy offload - anything I forgot? I see.
Re: [Cluster-devel] [Lsf-pc] [LSF/MM ATTEND] [TOPIC] fs/block interface discussions
On Wed, Dec 10, 2014 at 07:46:51PM +0100, Jan Kara wrote: But still you first need to stop all writes to the filesystem, then do a sync, and then allow writing again - which is exactly what freeze does. And with device-mapper, we were asked to support the taking of snapshots of multiple volumes simultaneously (e.g. where the application data is stored across more than one filesystem). Thin dm snapshots can handle this (the original non-thin ones can't). Alasdair
