Doesn't the existence of LVM snapshots slows down writing a file system wich doesn't support them natively?
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As far as understand snapshots in LVM (please, do correct me if I'm wrong): since they are not persistent and work even with a file system which doesn't itself support snapshots => I suppose it must mean that as soon as a snapshot is active, LVM will takes a copy of every block which is written to, before it is changed; this copy is saved to a RAM cache and eventually ends-up in another disk-space; and each read from the snapshot will be diverted to this «cache» if it exists there.
So I understand it means it should slow down every write while a snapshot exists. Does this mean that LVM snapshots should only be taken for as limited as possible durations, just for the time to backup data, and be suppressed as soon as possible ? And is this a concern only if the file system doesn't support snapshot natively ?
filesystems lvm cache snapshot copy-on-write
asked Mar 24 at 8:40
Camion
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up vote
0
down vote
favorite
As far as understand snapshots in LVM (please, do correct me if I'm wrong): since they are not persistent and work even with a file system which doesn't itself support snapshots => I suppose it must mean that as soon as a snapshot is active, LVM will takes a copy of every block which is written to, before it is changed; this copy is saved to a RAM cache and eventually ends-up in another disk-space; and each read from the snapshot will be diverted to this «cache» if it exists there.
So I understand it means it should slow down every write while a snapshot exists. Does this mean that LVM snapshots should only be taken for as limited as possible durations, just for the time to backup data, and be suppressed as soon as possible ? And is this a concern only if the file system doesn't support snapshot natively ?
filesystems lvm cache snapshot copy-on-write
asked Mar 24 at 8:40
Camion
62
add a comment |Â
up vote
0
down vote
favorite
up vote
0
down vote
favorite
As far as understand snapshots in LVM (please, do correct me if I'm wrong): since they are not persistent and work even with a file system which doesn't itself support snapshots => I suppose it must mean that as soon as a snapshot is active, LVM will takes a copy of every block which is written to, before it is changed; this copy is saved to a RAM cache and eventually ends-up in another disk-space; and each read from the snapshot will be diverted to this «cache» if it exists there.
So I understand it means it should slow down every write while a snapshot exists. Does this mean that LVM snapshots should only be taken for as limited as possible durations, just for the time to backup data, and be suppressed as soon as possible ? And is this a concern only if the file system doesn't support snapshot natively ?
filesystems lvm cache snapshot copy-on-write
asked Mar 24 at 8:40
Camion
62
As far as understand snapshots in LVM (please, do correct me if I'm wrong): since they are not persistent and work even with a file system which doesn't itself support snapshots => I suppose it must mean that as soon as a snapshot is active, LVM will takes a copy of every block which is written to, before it is changed; this copy is saved to a RAM cache and eventually ends-up in another disk-space; and each read from the snapshot will be diverted to this «cache» if it exists there.
So I understand it means it should slow down every write while a snapshot exists. Does this mean that LVM snapshots should only be taken for as limited as possible durations, just for the time to backup data, and be suppressed as soon as possible ? And is this a concern only if the file system doesn't support snapshot natively ?
filesystems lvm cache snapshot copy-on-write
asked Mar 24 at 8:40
Camion
62
asked Mar 24 at 8:40
Camion
62
asked Mar 24 at 8:40
Camion
62
asked Mar 24 at 8:40
Camion
62
62
add a comment |Â
add a comment |Â
1 Answer
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In short, yes, changing content which exists in a snapshot requires LVM to copy the content, which does slow down writes.
However, this happens regardless of the filesystem used because LVM operates at the block layer, and thus is unaware of the filesystem.
As for whether you should keep snapshots for only a short duration, that really depends on what is best for your use-case.
answered Mar 24 at 19:40
Emmanuel Rosa
2,1951410
What I mean is that if would require to keep snaphots for a longer duration as for having a way to rollback wrong changes, It might mean that I should favor another snapshot system (like one implemented at the file system level with a permanent copy on write strategy.);
– Camion
Mar 25 at 10:50
By the way, are you really sure that LVM is totally unaware of the file system ? Because il it's true, then it's even worse than I thought, since it means that it will even take copies of previously unallocated blocks.
– Camion
Mar 25 at 10:53
LVM uses Linux's device-mapper, which makes it completely unaware of the filesystem. See unix.stackexchange.com/questions/391998/…
– Emmanuel Rosa
Mar 25 at 21:34
add a comment |Â
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
0
down vote
In short, yes, changing content which exists in a snapshot requires LVM to copy the content, which does slow down writes.
However, this happens regardless of the filesystem used because LVM operates at the block layer, and thus is unaware of the filesystem.
As for whether you should keep snapshots for only a short duration, that really depends on what is best for your use-case.
answered Mar 24 at 19:40
Emmanuel Rosa
2,1951410
What I mean is that if would require to keep snaphots for a longer duration as for having a way to rollback wrong changes, It might mean that I should favor another snapshot system (like one implemented at the file system level with a permanent copy on write strategy.);
– Camion
Mar 25 at 10:50
By the way, are you really sure that LVM is totally unaware of the file system ? Because il it's true, then it's even worse than I thought, since it means that it will even take copies of previously unallocated blocks.
– Camion
Mar 25 at 10:53
LVM uses Linux's device-mapper, which makes it completely unaware of the filesystem. See unix.stackexchange.com/questions/391998/…
– Emmanuel Rosa
Mar 25 at 21:34
add a comment |Â
up vote
0
down vote
In short, yes, changing content which exists in a snapshot requires LVM to copy the content, which does slow down writes.
However, this happens regardless of the filesystem used because LVM operates at the block layer, and thus is unaware of the filesystem.
As for whether you should keep snapshots for only a short duration, that really depends on what is best for your use-case.
answered Mar 24 at 19:40
Emmanuel Rosa
2,1951410
What I mean is that if would require to keep snaphots for a longer duration as for having a way to rollback wrong changes, It might mean that I should favor another snapshot system (like one implemented at the file system level with a permanent copy on write strategy.);
– Camion
Mar 25 at 10:50
By the way, are you really sure that LVM is totally unaware of the file system ? Because il it's true, then it's even worse than I thought, since it means that it will even take copies of previously unallocated blocks.
– Camion
Mar 25 at 10:53
LVM uses Linux's device-mapper, which makes it completely unaware of the filesystem. See unix.stackexchange.com/questions/391998/…
– Emmanuel Rosa
Mar 25 at 21:34
add a comment |Â
up vote
0
down vote
up vote
0
down vote
In short, yes, changing content which exists in a snapshot requires LVM to copy the content, which does slow down writes.
However, this happens regardless of the filesystem used because LVM operates at the block layer, and thus is unaware of the filesystem.
As for whether you should keep snapshots for only a short duration, that really depends on what is best for your use-case.
answered Mar 24 at 19:40
Emmanuel Rosa
2,1951410
In short, yes, changing content which exists in a snapshot requires LVM to copy the content, which does slow down writes.
However, this happens regardless of the filesystem used because LVM operates at the block layer, and thus is unaware of the filesystem.
As for whether you should keep snapshots for only a short duration, that really depends on what is best for your use-case.
answered Mar 24 at 19:40
Emmanuel Rosa
2,1951410
answered Mar 24 at 19:40
Emmanuel Rosa
2,1951410
answered Mar 24 at 19:40
Emmanuel Rosa
2,1951410
answered Mar 24 at 19:40
Emmanuel Rosa
2,1951410
2,1951410
What I mean is that if would require to keep snaphots for a longer duration as for having a way to rollback wrong changes, It might mean that I should favor another snapshot system (like one implemented at the file system level with a permanent copy on write strategy.);
– Camion
Mar 25 at 10:50
By the way, are you really sure that LVM is totally unaware of the file system ? Because il it's true, then it's even worse than I thought, since it means that it will even take copies of previously unallocated blocks.
– Camion
Mar 25 at 10:53
LVM uses Linux's device-mapper, which makes it completely unaware of the filesystem. See unix.stackexchange.com/questions/391998/…
– Emmanuel Rosa
Mar 25 at 21:34
add a comment |Â
What I mean is that if would require to keep snaphots for a longer duration as for having a way to rollback wrong changes, It might mean that I should favor another snapshot system (like one implemented at the file system level with a permanent copy on write strategy.);
– Camion
Mar 25 at 10:50
By the way, are you really sure that LVM is totally unaware of the file system ? Because il it's true, then it's even worse than I thought, since it means that it will even take copies of previously unallocated blocks.
– Camion
Mar 25 at 10:53
LVM uses Linux's device-mapper, which makes it completely unaware of the filesystem. See unix.stackexchange.com/questions/391998/…
– Emmanuel Rosa
Mar 25 at 21:34
What I mean is that if would require to keep snaphots for a longer duration as for having a way to rollback wrong changes, It might mean that I should favor another snapshot system (like one implemented at the file system level with a permanent copy on write strategy.);
– Camion
Mar 25 at 10:50
What I mean is that if would require to keep snaphots for a longer duration as for having a way to rollback wrong changes, It might mean that I should favor another snapshot system (like one implemented at the file system level with a permanent copy on write strategy.);
– Camion
Mar 25 at 10:50
By the way, are you really sure that LVM is totally unaware of the file system ? Because il it's true, then it's even worse than I thought, since it means that it will even take copies of previously unallocated blocks.
– Camion
Mar 25 at 10:53
By the way, are you really sure that LVM is totally unaware of the file system ? Because il it's true, then it's even worse than I thought, since it means that it will even take copies of previously unallocated blocks.
– Camion
Mar 25 at 10:53
LVM uses Linux's device-mapper, which makes it completely unaware of the filesystem. See unix.stackexchange.com/questions/391998/…
– Emmanuel Rosa
Mar 25 at 21:34
LVM uses Linux's device-mapper, which makes it completely unaware of the filesystem. See unix.stackexchange.com/questions/391998/…
– Emmanuel Rosa
Mar 25 at 21:34
add a comment |Â
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