All this talk about controller, sync, buffer, storage, cache got me thinking.
I looked up out ZFS handles cache flushing, and how VirtualBox handles cache flushing. *According to http://docs.oracle.com/cd/E26505_01/html/E37386/chapterzfs-6.html <http://docs.oracle.com/cd/E26505_01/html/E37386/chapterzfs-6.html>* ZFS issues *infrequent flushes *(every 5 second or so) after the uberblock > updates. The flushing infrequency is fairly inconsequential so no tuning is > warranted here. ZFS also issues a flush every time an application requests > a synchronous write (O_DSYNC, fsync, NFS commit, and so on). > *According to http://www.virtualbox.org/manual/ch12.html <http://www.virtualbox.org/manual/ch12.html>* 12.2.2. Responding to guest IDE/SATA flush requests > > If desired, the virtual disk images can be flushed when the guest issues > the IDE FLUSH CACHE command. Normally *these requests are ignored *for > improved performance. The parameters below are only accepted for disk > drives. They must not be set for DVD drives. > I'm going to enable cache flushing and see how that affects results On Tue, Apr 1, 2014 at 7:14 PM, Bayard Bell <buffer.g.overf...@gmail.com>wrote: > Could you explain how you're using VirtualBox and why you'd use a type 2 > hypervisor in this context? > > Here's a scenario where you really have to mind with hypervisors: ZFS > tells a virtualised controller that it needs to sync a buffer, and the > controller tells ZFS that all's well while perhaps requesting an async > flush. ZFS thinks it's done all the I/Os to roll a TXG to stable storage, > but in the mean time something else crashes and whoosh go your buffers. > > I'm not sure it's come across particularly well in this thread, but ZFS > doesn't and can't cope with hardware that's so unreliable that it tells > lies about basic things, like whether your writes have made it to stable > storage, or doesn't mind the shop, as is the case with non-ECC memory. It's > one thing when you have a device reading back something that doesn't match > the checksum, but it gets uglier when you've got a single I/O path and a > controller that seems to write the wrong bits in stride (I've seen this) or > when the problems are even closer to home (and again I emphasise RAM). You > may not have problems right away. You may have problems where you can't > tell the difference, like flipping bits in data buffers that have no other > integrity checks. But you can run into complex failure scenarios where ZFS > has to cash in on guarantees that were rather more approximate than what it > was told, and then it may not be a case of having some bits flipped in > photos or MP3s but no longer being able to import your pool or having > someone who knows how to operate zdb do some additional TXG rollback to get > your data back after losing some updates. > > I don't know if you're running ZFS in a VM or running VMs on top of ZFS, > but either way, you probably want to Google for "data loss" "VirtualBox" > and whatever device you're emulating and see whether there are known > issues. You can find issue reports out there on VirtualBox data loss, but > working through bug reports can be challenging. > > Cheers, > Bayard > > On 1 April 2014 16:34, Eric Jaw <naisa...@gmail.com> wrote: > >> >> >> On Tuesday, April 1, 2014 7:04:39 AM UTC-4, jasonbelec wrote: >>> >>> ZFS is lots of parts, in most cases lots of cheap unreliable parts, >>> refurbished parts, yadda yadda, as posted on this thread and many, many >>> others, any issues are probably not ZFS but the parts of the whole. Yes, it >>> could be ZFS, after you confirm that all the parts ate pristine, maybe. >>> >> >> I don't think it's ZFS. ZFS is pretty solid. In my specific case, I'm >> trying to figure out why VirtualBox is creating these issues. I'm pretty >> sure that's the root cause, but I don't know why yet. So I'm just >> speculating at this point. Of course, I want to get my ZFS up and running >> so I can move on to what I really need to do, so it's easy to jump on a >> conclusion about something that I haven't thought of in my position. Hope >> you can understand >> >> >>> >>> My oldest system running ZFS is an Mac Mini Intel Core Duo with 3GB RAM >>> (not ECC) it is the home server for music, tv shows, movies, and some >>> interim backups. The mini has been modded for ESATA and has 6 drives >>> connected. The pool is 2 RaidZ of 3 mirrored with copies set at 2. Been >>> running since ZFS was released from Apple builds. Lost 3 drives, eventually >>> traced to a new cable that cracked at the connector which when hot enough >>> expanded lifting 2 pins free of their connector counter parts resulting in >>> errors. Visually almost impossible to see. I replaced port multipliers, >>> Esata cards, RAM, mini's, power supply, reinstalled OS, reinstalled ZFS, >>> restored ZFS data from backup, finally to find the bad connector end one >>> because it was hot and felt 'funny'. >>> >>> Frustrating, yes, educational also. The happy news is, all the data was >>> fine, wife would have torn me to shreds if photos were missing, music was >>> corrupt, etc., etc.. And this was on the old out of date but stable ZFS >>> version we Mac users have been hugging onto for dear life. YMMV >>> >>> Never had RAM as the issue, here in the mad science lab across 10 >>> rotating systems or in any client location - pick your decade. However I >>> don't use cheap RAM either, and I only have 2 Systems requiring ECC >>> currently that don't even connect to ZFS as they are both XServers with >>> other lives. >>> >>> >>> -- >>> Jason Belec >>> Sent from my iPad >>> >>> On Apr 1, 2014, at 12:13 AM, Daniel Becker <razz...@gmail.com> wrote: >>> >>> On Mar 31, 2014, at 7:41 PM, Eric Jaw <nais...@gmail.com> wrote: >>> >>> I started using ZFS about a few weeks ago, so a lot of it is still new >>> to me. I'm actually not completely certain about "proper procedure" for >>> repairing a pool. I'm not sure if I'm supposed to clear the errors after >>> the scrub, before or after (little things). I'm not sure if it even >>> matters. When I restarted the VM, the checksum counts cleared on its own. >>> >>> >>> The counts are not maintained across reboots. >>> >>> >>> On the first scrub it repaired roughly 1.65MB. None on the second scub. >>> Even after the scrub there were still 43 data errors. I was expecting they >>> were going to go away. >>> >>> >>> errors: 43 data errors, use '-v' for a list >>> >>> >>> What this means is that in these 43 cases, the system was not able to >>> correct the error (i.e., both drives in a mirror returned bad data). >>> >>> >>> This is an excellent question. They're in 'Normal' mode. I remember >>> looking in to this before and decided normal mode should be fine. I might >>> be wrong. So thanks for bringing this up. I'll have to check it out again. >>> >>> >>> The reason I was asking is that these symptoms would also be consistent >>> with something outside the VM writing to the disks behind the VM’s back; >>> that’s unlikely to happen accidentally with disk images, but raw disks are >>> visible to the host OS as such, so it may be as simple as Windows deciding >>> that it should initialize the “unformatted” (really, formatted with an >>> unknown filesystem) devices. Or it could be a raid controller that stores >>> its array metadata in the last sector of the array’s disks. >>> >>> >>> memtest86 and memtest86+ for 18 hours came out okay. I'm on my third >>> scrub and the number or errors has remained at 43. Checksum errors continue >>> to pile up as the pool is getting scrubbed. >>> >>> I'm just as flustered about this. Thanks again for the input. >>> >>> >>> Given that you’re seeing a fairly large number of errors in your scrubs, >>> the fact that memtest86 doesn’t find anything at all very strongly suggests >>> that this is not actually a memory issue. >>> >>> -- >> >> --- >> You received this message because you are subscribed to the Google Groups >> "zfs-macos" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to zfs-macos+unsubscr...@googlegroups.com. >> >> For more options, visit https://groups.google.com/d/optout. >> > > -- > > --- > You received this message because you are subscribed to a topic in the > Google Groups "zfs-macos" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/zfs-macos/qguq6LCf1QQ/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > zfs-macos+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- --- You received this message because you are subscribed to the Google Groups "zfs-macos" group. 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