understanding the tree-log

understanding the tree-log

[Phillip Susi]

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Given how a transaction is committed, why is there a tree-log?  When
modifying the tree, either the super block is still pointing to the
old tree root, or the new generation has been fully committed, so how
can there ever be an intermediate state that needs a log to recover?
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Re: understanding the tree-log

[Liu Bo]

On 11/04/2011 11:21 AM, Phillip Susi wrote:
> Given how a transaction is committed, why is there a tree-log?  When
> modifying the tree, either the super block is still pointing to the
> old tree root, or the new generation has been fully committed, so how
> can there ever be an intermediate state that needs a log to recover?

tree-log is for improving the preformance  of fsync and and O_SYNCs.

Btrfs has an expensive "commit transaction", if we commit a transaction every time we fsync,
the performance is not that good.  Instead of this, we introduce a write-ahead log to make
our fsync faster.

So if you do fsync for your data, it means your data is safely in the log tree,
then if a crash takes place, the data can be recovered from log.

More details are in tree-log.c.

thanks,
liubo

Re: understanding the tree-log

[Phillip Susi]

On 11/4/2011 1:09 AM, Liu Bo wrote:
> Btrfs has an expensive "commit transaction", if we commit a transaction every time we fsync,
> the performance is not that good.  Instead of this, we introduce a write-ahead log to make
> our fsync faster.
>
> So if you do fsync for your data, it means your data is safely in the log tree,
> then if a crash takes place, the data can be recovered from log.

How can you write to the log tree without a full commit?  The tree of 
tree roots has to point to the root node of the log tree, so when you 
write out the log tree, that needs updated too, which requires a full 
commit doesn't it?