[Nottingham] Copy without copying? (file copy deduplication)

VM vadim at mankevich.co.uk
Mon Jun 10 13:28:02 UTC 2019

Martin, LVM snapshots can be mounted read-write and keep modifications made. They also can be merged with the origin if changes are desired. For instance, make a snapshot, mount it, upgrade, check everything is fine, merge snapshot with the origin. It's another use of LVM snapshots, different to read-only backups.

On June 10, 2019 12:37:55 PM UTC, Martin via Nottingham <nottingham at mailman.lug.org.uk> wrote:
>On 10/06/2019 12:06, J J via Nottingham wrote:
>> I must be misunderstanding something, I thought the snapshot retained
>> the original data as things continued to get written to the source.
>> (This seems to be what LVM does at any rate)
>On LVM, when you enable a snapshot device:
>The source device is used as normal, unaffected, with data read and new
>data written as normal.
>While the LVM snapshot device is enabled, any disk blocks written to
>source device first have the old block data copied to the snapshot
>device. Hence you get the original LVM source device in use as normal
>and a copy of the old data before being overwritten, block by block,
>copied across to the LVM snapshot device. I guess that can be called a
>sort of CoW but used instead for 'saving' the old data.
>When reading from the LVM snapshot device, any blocks that are
>are read from the source device as normal. Any changed blocks are
>instead read from the copy saved on the snapshot device to make the
>source device appear to be unchanged.
>Hence, you have the read-only snapshot view and additionally, the
>snapshot device area can be much smaller than the source device. (Only
>the changed data is stored...)
>> Which is great, but kinda the wrong way round for my use case.
>> I effectively want the changes written into the snapshot, leaving the
>> source untouched.
>> Thus I can have as many snapshots as I need (parallel tests etc) and
>> then chuck them.
>> Much more like the differencing you get with Hypervisors (and
>> just at the block level to reduce space requirements).
>And that is exactly what you can do on btrfs with the inherent magic of
>Simplistically, all a btrfs snapshot is is an atomic snapshot of the
>b-tree indexing. The file data stays in place.
>You then have, ready for the very next operation, two identical views
>the same data and attributes. There is no distinction as to which is a
>snapshot of whichever. You have taken ' _a_ snapshot' whereby you now
>have two identical views with access from two different points on the
>Both views are read-writeable in whatever way you wish.
>Or you can restrict whichever or both to read-only in whatever way
>> If XFS or something can do that, awesome, and I'll start looking at
>> as soon as Kubuntu gets itself out of whatever APT fankle it's
>> to get into!
>No need for XFS (and in any case, I'm a little prejudiced against its
>block allocation ways of working and how that must fit your use case -
>whereas btrfs is more flexible for extremes of use cases).
>> Not sure how viable BTRFS would be as a choice, I thought the project
>> was slowly dying off (I know RedHat has dropped it) and any proposals
>> will need to be passed by Internal IT, so can't be too exotic.
>btrfs is very much alive and is used by and supported by the big cloud
>providers including Facebook...
>Red Hat only dropped support for some of their special Red Hat kernel
>versions for the sake of long term maintainability.
>btrfs is still developing apace for new features. However, their main
>features and disk structure can be considered long stable.
>I've been using btrfs for about a decade to good effect and without any
>adverse issues and spanning many TBytes of data.
>Hope that's of help,
>ps: Corrections welcomed!
>> On Mon, 10 Jun 2019 at 11:24, Martin via Nottingham
>> <nottingham at mailman.lug.org.uk
><mailto:nottingham at mailman.lug.org.uk>>
>> wrote:
>>     Jason,
>>     The obvious fix for that is to use btrfs whereby:
>>     You have your pristine folders in one btrfs subvolume (can even
>>     mounted or set to be read-only);
>>     Then create a snapshot subvolume from that first subvolume;
>>     Your new snapshot subvolume can then be used or mounted
>>     Aside: Note that you don't even need to mount the subvolumes as
>>     mounts: they can even appear as 'normal' directories on a
>>     higher mount point.
>>     The snapshot volume is created with zero copying of files and is
>>     fast and low resource to do. You'll see no increase in disk
>>     During use, the only file writing will be only for whatever new
>data is
>>     actually written. The magic of the btrfs CoW means that only new
>>     of data get written. Unchanged file fragments remain unchanged
>>     uncopied...
>>     Hope that fits your needs?
>>     Cheers,
>>     Martin
>>     On 10/06/2019 11:04, J J via Nottingham wrote:
>>     > I need read/write access to the new folder.
>>     > Mucking around with OverlayFS I can effectively have a
>>     folder
>>     > and then a "Working" folder (plus a couple of others for
>reasons) and
>>     > that works well. Except for the fact that full file copies
>>     happen
>>     > and some of these files are big.
>>     > So the situation is definitely better, but far from ideal.
>>     >
>>     > I am going to see if I can grok XFS enough to do the similar
>>     thing, but
>>     > do some block-level magics.
>>     > LVM snapshots might also do it, but not sure if they would
>allow for
>>     > multiple, concurrent "Working" folders and AIUI that would mean
>>     mucking
>>     > around with how the system is deployed, not something I have
>>     control over.
>>     >
>>     > The other wrinkle is that this needs t be done ad hoc.
>>     > But first things first...
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