Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
> On Thu, Dec 13, 2007 at 02:00:05PM +0100, Mark Hunting wrote: > >> Hi all, >> >> I have set up DRBD (0.7.21, Debian Etch) on two storage systems. Both >> systems use a Areca Raid controller with 1GB cache memory. When I copy a >> file to the DRBD device on the primary, the copying finishes with a >> speed of about 400MB per second. That's should be impossible, DRBD uses >> a gigabit link to synchronize, the copying should proceed with a rate op >> 100 MB per second (drbd.conf: rate 100M). >> >> So apparently the copying finishes before the data is transferred to the >> secondary. Indeed I still see a lot of DRBD network traffic for some >> seconds after the copy command finishes. This should be impossible with >> DRBD, the copying should only finish when the data is both written on >> the primary AND the secondary, right? When the primary server >> unexpectedly goes down right after a file has been copied and the >> secondary server takes over, the copied file is not there. >> I think this has to do with the 1GB cache memory on the Areca cards, >> when I transfer files that are much bigger than 1GB the copying indeed >> finishes with a speed close to 100MB per second. >> >> I don't understand this, what difference does it make for DRBD what is >> underneath the DRBD device (/dev/drbd0)? It should not matter if there >> is only one hard disk, a RAID system, a RAID system with cache memory or >> whatever. But apparently it does matter in my case. >> >> Is this a known problem, and how can I avoid it? Right now I can never >> be sure my two storage systems are fully synchronized. >> > > man fsync > man fdatasync > man sync > > wikipedia page_cache > Yes I understand that the page cache can speed things up a lot. But how is it possible that when I copy a new file to the DRBD disk the copying is much faster than the DRBD link of 100MB/second. To be clear, with 'new' file I mean a file that I have never used on the DRBD disk before. So it can not be in the cache of the secondary server. Therefore the copying should go with a rate of 100MB/second, not faster like it does now. Or am I missing something here? Mark