Chris Mason wrote:
On Thu, Jun 04, 2009 at 10:49:19AM +0200, Thomas Glanzmann wrote:
Hello Chris,
My question is now, how often can a block in btrfs be refferenced?
The exact answer depends on if we are referencing it from a single
file or from multiple files. But either way it is roughly 2^32.
could you please explain to me what underlying datastructure is used to
monitor if the block is still referenced or already free? Is a counter
used, bitmap (but that can't be if is 2^32) or some sort of list? I
assume that a counter is used. If this is the case, I assume when a
snapshot for example is deleted the reference counter of every block
that was referenced in the snapshot will be decremented by one. Is this
correct or am I missing something here?
It is a counter and a back reference. With Yan Zheng's new format work,
the limit is not 2^64.
When a snapshot is deleted, the btree is walked to efficiently drop the
references on the blocks it referenced.
From a dedup point of view, we'll want the dedup file to hold a
reference on the file extents. The kernel ioctl side of things will
take care of that part.
I wonder how well would deduplication work with defragmentation? One
excludes the other to some extent.
--
Tomasz Chmielewski
http://wpkg.org
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