Re: Why we need delayed allocation!

that's very good point. thanks for notice and explanation.
at present, I use ->writepages(), not ->writepage() to find/allocate
extents. judging from sources, ->writepages() isn't used during memory
pressure.

>>>>> Nikita Danilov (ND) writes:

 ND> One lesson we learned in reiser4 is that delayed allocation done at the
 ND> memory pressure (that is, from ->writepage()) doesn't play well with
 ND> extents. If file system uses extents to keep track of where file's
 ND> blocks are, and extents are initially "unallocated" then at the
 ND> -> writepage() time one has to allocate them. This leads to all sorts of
 ND> nasty worst cases and deadlocks, because during allocation single
 ND> unallocated extent can be split into many allocated ones (due to bitmap
 ND> fragmentation), which requires insertion of these new extents, which is
 ND> too late to do safely at the ->writepage() time, because you are out of
 ND> memory already.




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