On Sun, 25 Nov 2012 21:48:09 +0200 Toomas Aas <[email protected]> wrote:
> > Why doesn't Amanda write these 5 DLEs to tape while dumping the 6th > one? The tape drive isn't doing anything (amstatus shows 'Taper > status: idle'). I can't see any obvious parameters in amanda.conf > that would control this. The current behaviour - first writing > (almost) all the DLEs to holding disk and then from holdingdisk to > tape - means that amdump run takes much longer compared to if the > 'wait-for-writing-to-tape' DLEs were taped simultaneously with > dumping these DLEs that are later in the queue. I'll take a guess, and others may jump in to tell me where I am wrong. One problem with physical tapes is that stopping and then starting up immediately is that this takes a *long* time. Stopping is not instantaneous. Then, to restart where you left off involves backing up and restarting, reading until you get to where you left off. It's also rough on the tape and the drive. A lot of that in a short period of time we call "shoe shining". Imagine the head as the shoe. So you want to avoid stopping and starting. The simplest way to do so is to wait until all the dumps are complete, then tossing them all onto the tape drive. This assumes the holding disk is big enough. Amanda is a bit smarter than that; it waits until it has enough on the holding disk that it is a good bet it won't have to stop and start. I don't know the exact algorithm Amanda uses, but I expect it is selected to give you continuous writing once writing has started, even if it means a longer run time. > > If it matters, this is Amanda 3.2.0 using vtapes with chg-disk. vtapes emulate physical tapes. And they are fast compared to physical tape. So I suspect Amanda would wait longer to start writing than if it were using a slow physical tape. -- Charles Curley /"\ ASCII Ribbon Campaign Looking for fine software \ / Respect for open standards and/or writing? X No HTML/RTF in email http://www.charlescurley.com / \ No M$ Word docs in email Key fingerprint = CE5C 6645 A45A 64E4 94C0 809C FFF6 4C48 4ECD DFDB
