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Nar
n00b
Joined: 27 May 2002
Posts: 49
Location: Sandton, South Africa
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 Posted: Wed Oct 23, 2002 8:56 am Post subject: Questions about EVMS ? |
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Hi Gentoo People
Got a few questions about EVMS ?
1. How does the snapshot feauter work in EVMS ? I want to know how they do it 
2. Is it posiible to do a snapshot of only file that have changed ?
Other wise evms rock
Thanks
Nar |
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chavez
n00b
Joined: 29 May 2002
Posts: 35
Location: Austin, Texas
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| Posted: Fri Oct 25, 2002 2:41 am Post subject: Re: Questions about EVMS ? |
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| Nar wrote: | Hi Gentoo People
Got a few questions about EVMS ?
1. How does the snapshot feauter work in EVMS ? I want to know how they do it
2. Is it posiible to do a snapshot of only file that have changed ?
Other wise evms rock
Thanks
Nar |
A highlevel answer is that the snapshot feature plug-in hooks all toplevel I/O requests being directed at a volume being snapshotted. If it is a write, the snapshot plugin issues a read for the original data and writes the original data to the snapshot backing store volume. It then updates its mapping tables to indicate the mapped blocks. When reading from snapshot volume (the backing store), unchanged blocks are read from the original and changed blocks are read from the backing store. If you need more detail, let me know.
The EVMS Snapshot feature snapshots volumes regardless of the filesystem. So the snapshot is not at a file level per se but a device/volume block level. But to answer your question, if any block on the volume being snapshotted changes, the original data is copied to the backing store. So the backing store is not the same a full mirror. Because it uses a copy-on-write approach, the backing store snapshot volume can be smaller than the volume being snapshotted if not many changes are expected to occur during the existance of the snapshot volume.
I agree, "EVMS rox!".
_________________
regards,
Luciano Chavez |
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Nar
n00b
Joined: 27 May 2002
Posts: 49
Location: Sandton, South Africa
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| Posted: Fri Oct 25, 2002 6:06 am Post subject: |
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Thank you for the reply.
I just made a mistake on my second question :\ Let me ask it again
Is it possible to make only a snapshot of the files that changed since your last snapshot. What I want to do is make every day a snapshot and keep it for two days. To take a snapshot of everything will make it a space problem. So it will be better to take a snapshot of only the change since the last snapshot Is this possible ?
Thanks
Nar |
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chavez
n00b
Joined: 29 May 2002
Posts: 35
Location: Austin, Texas
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| Posted: Fri Oct 25, 2002 6:57 pm Post subject: |
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| Nar wrote: | Thank you for the reply.
I just made a mistake on my second question :\ Let me ask it again
Is it possible to make only a snapshot of the files that changed since your last snapshot. What I want to do is make every day a snapshot and keep it for two days. To take a snapshot of everything will make it a space problem. So it will be better to take a snapshot of only the change since the last snapshot Is this possible ?
Thanks
Nar |
Nar,
Well, a snapshot is simply that, a snapshot of a volume in time. It is allowable to have multiple snapshots of the same volume. So for example you could create a snapshot for a volume today. Tomorrow, you could create a new snapshot for the same volume to essentially be a snapshot of changes that occurred since yesterday. That way you could mount the two snapshot volumes to determine differences. Is this sort of what you are looking for?
_________________
regards,
Luciano Chavez |
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