On 07/21/09 01:21 PM, Richard Elling wrote:
I never win the lottery either :-)
Let's see. Your chance of winning a 49 ball lottery is apparently around 1 in 14*10^6, although it's much better than that because of submatches (smaller payoffs for matches on less than 6 balls). There are about 32*10^6 seconds in a year. If ZFS saves its writes for 30 seconds and batches them out, that means 1 write leaves the buffer exposed for roughly one millionth of a year. If you have 4GB of memory, you might get 50 errors a year, but you say ZFS uses only 1/10 of this for writes, so that memory could see 5 errors/year. If your single write was 1/70th of that (say around 6 MB), your chance of a hit is around 5/70/10^-6 or 1 in 14*10^6, so you are correct! So if you do one 6MB write/year, your chances of a hit in a year are about the same as that of winning a grand slam lottery. Hopefully not every hit will trash a file or pool, but odds are that you'll do many more writes than that, so on the whole I think a ZFS hit is quite a bit more likely than winning the lottery each year :-). Conversely, if you average one big write every 3 minutes or so (20% occupancy), odds are almost certain that you'll get one hit a year. So some SOHO users who do far fewer writes won't see any hits (say) over a 5 year period. But some will, and they will be most unhappy -- calculate your odds and then make a decision! I daresay the PC makers have done this calculation, which is why PCs don't have ECC, and hence IMO make for insufficiently reliable servers. Conclusions from what I've gleaned from all the discussions here: if you are too cheap to opt for mirroring, your best bet is to disable checksumming and set copies=2. If you mirror but don't have ECC then at least set copies=2 and consider disabling checksums. Actually, set copies=2 regardless, so that you have some redundancy if one half of the mirror fails and you have a 10 hour resilver, in which time you could easily get a (real) disk read error. It seems to me some vendor is going to cotton onto the SOHO server problem and make a bundle at the right price point. Sun's offerings seem unfortunately mostly overkill for the SOHO market, although the X4140 looks rather interesting... Shame there aren't any entry level SPARCs any more :-(. Now what would doctors' front offices do if they couldn't blame the computer for being down all the time?
It is quite simple -- ZFS sent the flush command and VirtualBox ignored it. Therefore the bits on the persistent store are consistent.
But even on the most majestic of hardware, a flush command could be lost, could it not? An obvious case in point is ZFS over iscsi and a router glitch. But the discussion seems to be moot since CR 6667683 is being addressed. Now about those writes to mirrored disks :) Cheers -- Frank _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
