zfs discuss - Mar 2009 - Forensics related ZFS questions

1. Does variable FSB block sizing extend to files larger than record
size, concerning the last FSB allocated?
 
In other words, for files larger than 128KB, that utilize more than one
full recordsize FSB, will the LAST FSB allocated be
''right-sized'' to fit
the remaining data, or will ZFS allocate a full recordsize FSB for the
last ''chunk'' of the file?  (This is a file slack issue re: how
much will
exist.)
 
2. Can a developer confirm that COW occurs at the FSB level (vs. sector
level, for example)? 
 
In other words, when a single FSB (say 64KB file w/ recordsize=128KB)
file is modified, and it''s only one sector within that file
that''s
modified, is it correct that what''s copied-on-write is the entire 64KB
FSB allocated to that file?  (This is a data recovery issue.)
 
 
 
NICOLE L. BEEBE, Ph.D., CISSP
Assistant Professor
The University of Texas at San Antonio
Department of Information Systems & Technology Management
 
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>    1. Does variable FSB block sizing extend to files larger than record
size,
>    concerning the last FSB allocated?
> 
>    In other words, for files larger than 128KB, that utilize more than one
>    full recordsize FSB, will the LAST FSB allocated be
`right-sized'' to fit
>    the remaining data, or will ZFS allocate a full recordsize FSB for the
>    last `chunk'' of the file?
The last block is currently a multiple of the recordsize, but we intend
to fix this.  There are two options: one, to treat the last block as a
special case; the other, to handle it automatically via compression.
The former is a little more work, but has the advantage of reducing
the file''s in-memory footprint as well as its on-disk footprint.
>    2. Can a developer confirm that COW occurs at the FSB level (vs. sector
>    level, for example)?
> 
>    In other words, when a single FSB (say 64KB file w/ recordsize=128KB)
file
>    is modified, and it''s only one sector within that file
that''s modified, is
>    it correct that what''s copied-on-write is the entire 64KB FSB
allocated to
>    that file?  (This is a data recovery issue.)
Yes, that''s correct.

Jeff