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Kirill Smelkov
wendelin.core
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45db6007
Commit
45db6007
authored
Sep 24, 2015
by
Kirill Smelkov
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bigfile/zodb: General overview how virtmem -> ZODB propagation works
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65f6a5db
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bigfile/file_zodb.py
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45db6007
...
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@@ -50,14 +50,30 @@ will be our future approach after we teach NEO about object deduplication.
~~~~
TODO big picture module description
As file pages are changed in RAM with changes being managed by virtmem
subsystem, we need to propagate the changes to ZODB objects back at some time.
T
hings are done this way (vs more ZODB-like way) because
:
T
wo approaches exist
:
- compatibility with FS approach (to be able to switch to fuse)
- only one manager (vs two managers - bigfile & Connection and synchronizing them)
- on abort no need to synchronize changes (= "only 1 manager" as above)
- TODO ...
1) on every RAM page dirty, in a callback invoked by virtmem, mark
corresponding ZODB object as dirty, and at commit time, in
obj.__getstate__ retrieve memory content.
2) hook into commit process, and before committing, synchronize RAM page
state to ZODB objects state, propagating all dirtied pages to ZODB objects
and then do the commit process as usual.
"1" is more natural to how ZODB works, but requires tight integration between
virtmem subsystem and ZODB (to be able to receive callback on a page dirtying).
"2" is less natural to how ZODB works, but requires less-tight integration
between virtmem subsystem and ZODB, and virtmem->ZODB propagation happens only
at commit time.
Since, for performance reasons, virtmem subsystem is going away and BigFiles
will be represented by real FUSE-based filesystem with virtual memory being
done by kernel, where we cannot get callback on a page-dirtying, it is more
natural to also use "2" here.
"""
from
wendelin.bigfile
import
BigFile
,
WRITEOUT_STORE
,
WRITEOUT_MARKSTORED
...
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@@ -76,8 +92,6 @@ import os
# FIXME peak 2·Ndirty memory consumption on write (should be 1·NDirty)
# XXX write about why we can't memory-dirty -> ZBlk (memory could be modified
# later one more time)
# Base class for data of 1 file block as stored in ZODB
class
ZBlkBase
(
Persistent
):
...
...
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