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<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Hi,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>My company is investigating MogileFS and we are trying to
come up with a good method to rebalance files when new storage nodes are
added. Defining our own replication policy solves part of the problem. However,
ideally, we’d like to specify where the first copy of a new file is
stored in the same way that we are able to specify where subsequent copies are
stored (in the replication policy). We’d like to be able to define
our own Mgd::find_deviceid. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>To show why we’d need to define our own
Mgd::find_deviceid, this is how we’d implement rebalancing:<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>- Before existing storage nodes are too full (say at ~60%
capacity), add some new empty storage nodes,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>- Mgd::find_deviceid would specify to save the initial copy
of new files to existing storage nodes only. (Reason: we assume that new
files are more frequently accessed than older ones and do not want the majority
of new files on the new storage nodes).<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>- We would run a separate process that simply “rewrites”
old files. When “resaving” the initial copy of these old
files, Mgd::find_deviceid would specify to save them to the new storage
nodes. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>- For replication, our policy would use the same Mgd::find_deviceid
we defined above to replicate new files to existing storage nodes and old files
to new storage nodes.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>- Once all storage nodes are balanced, we will stop “rewriting”
old files and new files will be saved as they are now…to the storage node
with the most available space.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>- A few notes:<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'> - We are able to differentiate between new files and
rewritten files because we store meta data about each file outside of MogileFS
(such as the original creation date). If the creation date is before the
date the storage node was added, we assume it is being rewritten, otherwise, we
assume it is new.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'> - A storage node is considered “new” if
its usage is largely less than the average usage across all storage nodes.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>If we added a patch that allowed users to define their own version
of Mgd::find_deviceid, would that be included?<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Thanks,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><st1:City w:st="on"><st1:place w:st="on"><font size=2
face=Arial><span style='font-size:10.0pt;font-family:Arial'>Newton</span></font></st1:place></st1:City><font
size=2 face=Arial><span style='font-size:10.0pt;font-family:Arial'> Saunders<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
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