Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
Lars Ellenberg wrote: > > / 2004-01-22 11:01:58 -0500 > \ george young: > > On Thu, 22 Jan 2004 10:23:43 -0500 > > Todd Denniston <Todd.Denniston at ssa.crane.navy.mil> threw this fish to the penguins: > > > > > Hi, > > > [drbd-0.6.10, Fedora Core release 1, 2 node] > > > As I was beginning to create my drbd.conf, using the > > > /usr/share/doc/drbd-0.6.10/drbd.conf example, I read the following comment: > > > > > > # btw, don't do this. > > > # did you notice that in this example we have two drbd devices > > > # on the same spindle (hda)? performance will be bad. if you > > > # use several drbd devices, put them on different spindles; > > > # different channels/controllers won't be a bad idea for IDE. > > > > > > I have a raid box [Promise RM8K, RAID 5, ~750GB, SCSI], which presents itself > > > as one disk, AKA spindle, and I wanted to know if the above note was mainly > > now, if it has more than one physical spindle, it _has_ more than > one spindle. So the argument does not hold exactly. > <SNIP> > > > Is this because DRBD/EXT3 makes some assumptions about stacking up writes that > > > are not true when presented with multiple partitions, or just that the seek > > > times get amplified badly with drbd in protocol=C? > > it is mostly because of seektime killing your resynchronization performance. > you can avoid that nowadays by putting the devices in different "sync-group"s. > <SNIP> > you cannot have partitions in drbd. what you can do is have drbd as > a LVM2-PV. but you should know why you are doing it before you try this. > <SNIP> > > Also keep in mind that you are mirroring a block device, not a file system. > > DRBD doesn't know or care that you may only be using 20% of a given fs, > > it will still sync the whole device's contents across the net when it > > chooses. Work out how long it would take to copy your entire 750GB > > across your net... > > Yes. Don't mirror more than neccessary. I'd suggest to have a LVM > logical volume as the lower level device for drbd. If you later > figure you need more space, you can easily but *carefully* do > lvextend, change the disk-size in drbd.conf, e2fsck -f, resize2fs, > and be happy... > > Lars Ellenberg Thanks for the info, I want to re-state it to make sure I understand correctly. If I had more disks in the array (currently only 5) and it handled presenting multiple logical spindles, I MIGHT be better off doing it that way, however just having a multiple physical disk RAID mitigates the issues (some). With 'modern' drbd I can mitigate what was seen as the biggest issue, with single spindle multiple drbd, by making sure each partition (or logical volume) in a single array is in a different "sync-group". And yes I did not want to have to sync the whole array if I could keep from it, I even intend to have one partition read only 99% of the time because it only needs to be updated when I update shared application binaries. It is nice to hear that DRBD is working with LVM-2, it was something I had thought about in passing, but as I don't yet know LVM's I am going to attempt to put implementing that on a server, off to the next upgrade. -- Todd Denniston Crane Division, Naval Surface Warfare Center (NSWC Crane) Harnessing the Power of Technology for the Warfighter