[DRBD-user] DRBD8 Active/Active HOWTO

Craig O'Shannessy craig at ucw.com.au
Tue Jul 25 06:00:52 CEST 2006

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

I'm wondering if the new kernel option "drop_caches" would allow read 
only access to the secondary?


The reason I want this is for the following scenario

* Primary machine has a small DRDB partition with the Postgres "write 
ahead log" (perhaps stored in a battery backed RAM disk for performance)
* Secondary machine has a copy of the PG database, and, as long as it 
can READ the write ahead log mirror (DRBD or Raid1), it can keep a hot 
remote ACID compliant backup of the postgres database.

Something like this on the secondary

while true
   mount -o ro /mnt/pg_wal
   process new WAL entries (this is a read only op, on what is basically 
a sequential file that just gets added to by the primary)
   umount /mnt/pg_wal
   echo 3 > /proc/sys/vm/drop_caches # see below
   sleep 2

Here's the details from the above link, about the new proc option.  Can 
anyone give me any feedback on whether this is a viable option?


Writing to this will cause the kernel to drop clean caches, dentries and 
inodes from memory, causing that memory to become free.

To free pagecache:

    * echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes:

    * echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:

    * echo 3 > /proc/sys/vm/drop_caches

As this is a non-destructive operation and dirty objects are not 
freeable, the user should run "sync" first.

Dr. Volker Jaenisch wrote:
> Hi DRBD People!
> drbd0.7 is in production on some of our customers sites.
> Thank you very much for this fine pice of software.
> Now we are curious on DRBD8.
> We have a testing instance of drbd8-pre3 running smootly on two debian 
> etch servers ACTIVE/PASSIVE.
> Is there any documentation to setup ACTIVE/ACTIVE yet?
> Please give us some hints to satisfy our curiousity.
> Best Regards
> Volker

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