Mike Audia posted on Fri, 02 Aug 2013 16:58:42 -0400 as excerpted: >> From: David Sterba There were a few requests to tune the interval. This >> finally made me to finish the patch and will send it in a second. > > Thank you, David and to others who kindly replied to my post. I will > try your patch rather than modifying the code > >> > > Are there any unforeseen and effects of doing this? Thank you >> > > for >> > > the consideration. >> > >> > I don't *think* that there should be. One way of looking at it is >> > that both 30 and 300 seconds are an *eternity* for cpu, memory, and >> > storage. >> > Any trouble that you could get in to in 300 seconds some other >> > machine could trivially get in to in 30 with beefier hardware. >> >> That's a good point and lowers my worries a bit, though it would be >> interesting to see in what way a beefy machine blows with 300 seconds >> set. > > I have my system booting to a BTRFS root partition. Let's say I'm using > a value of 300 for my checkpoint interval. Does this mean that if I do > a TON of filesystem writes (say I update my system which pulls down a > bunch of system file updates for example), and I copy over several gigs > of data from a backup, all _between_ checkpoints and for some reason, my > system freezes forcing me to ungracefully restart... is EVERYTHING since > the last checkpoint is lost?
When I tried btrfs on faulty hardware a bit over a year ago, yes. And yes, that's the way a btree filesystem such as btrfs generally works, too, because when a change happens, it recurses up the tree until finally the master node is updated. Until the master node is updated, the old master node remains effective. During the time between the first change and the master node update, additional changes may occur, making the final master node update and likely several below it more "efficient", since that single write now covers more than a single change. However, if the system bellys up in the meantime, that means you lose everything since the last master node update. Here's my experience from last year. I had some failing hardware, which turned out to be the mobo, but before I ultimately figured out the problem, I thought it was the disks. Thus, I bought a new one and attempted to replace what I thought was a failing one, copying everything over, and thinking I'd try the new to me btrfs while I was at it. But what was really happening hardware-wise was that my then 8-year-old mobo had some capacitors going bad (I found several bulging and others burst when I finally figured out it was the mobo). That was triggering intermittent I/O errors that I had (wrongly) attributed to the disks dying, thus the replacement attempt. The symptom was SATA retries, downgrading the speed and retrying again, and eventually timing out and resetting the SATA interface. Only sometimes the whole system would lockup before a successful reset, or it would timeout and reset enough times that I'd give up and do a full system reset. The one thing I /did/ notice was that if I kept things cold enough (by the time I was done I had the AC turned down so far I was sitting here in a full winter jacket, long underwear, and a knit hat... in a Phoenix summer with temps of 40-45C/100-115F outside!!), the system would work better, so that's what I was trying to do. It was in this environment that I was attempting to copy all my old data from what I /thought/ was a failing disc drive (or drives, I was running md/raid1 for most of the system), initially blaming the copy failures on what I thought was the failing drive(s), until I had enough data on the new drive to try disconnecting the old drives and copying data around on the new drive. When that acted up with the old drives entirely disconnected, I realized it wasn't the drives after all, and eventually found the problem. But meanwhile, when I'd have to reset, what I'd find is that on btrfs, the whole tree I had been trying to copy over, and that I /thought/ had mostly copied fine, was gone. Or worse, part of the metadata had copied, the filesystem tree or at least part of it, and was still there after a reboot, but all or most of the files were zeroed out!! At least if nothing at all copied I knew right away where I was at. With the zeroed out files, I'd have to figure out how much actual data had copied and remained on the new drive, and where it had gone from saving everything to only saving the metadata, with the actual files zeroed out. Then I could delete them and try again. My previous filesystem (and the one I returned to for a year after I gave up on btrfs for the time being, I'm back on btrfs, with new SSDs, now) was reiserfs. It has actually been *IMPRESSIVELY* reliable for me, even thru various hardware failure, at least since the reiserfs data=ordered by default mode was introduced back in kernel 2.6.6 or some such. (As it turns out, it was the same Chris Mason working on that after Hans Reiser and Namesys basically abandoned reiserfs in favor of working on reiser4, that's behind btrfs now, so he knows his filesystems!) What I found is that with properly tuned vm.dirty_* as explained in my earlier post, or with repeatedly hitting the magic-SRQ emergency sync hotkey (alt-srq-s), reiserfs had a chance to lose *ONLY* the data that hadn't yet been synced, while btrfs would tend to either entirely lose or zero out the files for entire freshly copied trees, since the start of the copy operation, EVEN WHEN I HAD BEEN EMERGENCY SYNCING EVERY FEW SECONDS AS THE COPY PROGRESSED!! Obviously, then, btrfs simply wasn't going to work with my at the time bad hardware, certainly not for the massive data transfers I was trying to do, since with btrfs after a crash I had lost pretty much all of the current copy I had been doing, while reiserfs would reliably actually sync when I hit the emergency sync sequence, so after a crash I'd lose ONLY the few files since the last sync a few seconds before. Thus, even with failing hardware I could make reasonable progress with the copy when the destination was reiserfs, where with btrfs, in most cases I was back at square one, as if I'd never done that copy at all, or worse yet, with a bunch of zeroed out files where the metadata was retained but not the actual data. So I switched back to reiserfs for a year, and only tried btrfs again from a couple months ago now, when I upgraded to SSD and thus had both brand new and much faster hardware to worth with, AND needed a filesystem more suited to SSD than reiserfs. (I had found reiserfs MUCH better and more robust for my needs than ext3/4 back on spinning rust, and didn't really want to go ext4 on SSD either, tho I probably would have without btrfs where it is now.) In that year btrfs has GREATLY matured as well, and to be honest I'm not sure whether its btrfs increased maturity and stability over that year or the fact that I'm on actually GOOD hardware now that makes the btrfs experience so much better for me now, but regardless, btrfs IS still experimental, and even when that label comes off, I expect it'll take quite some time to reach the stability of current reiserfs, just as it took time for reiserfs to reach that. But of course btrfs is far more flexible than reiserfs as well, both SSD-wise and in general. Still, in a crash and DEFINITELY in the failing hardware scenario, I'd definitely put a lot more trust in reiserfs than in btrfs, and I expect it'll be that way for some time to come. > Upon a reboot, will BTRFS just mount up to > the last good checkpoiint automatically or will I have a broken system > and need to add the `-o recovery` option while I mount it manualy from a > chroot? In general btrfs should simply mount the last checkpoint automatically. And with a recently created filesystem I think it'll do pretty good at that. However, btrfs IS still experimental, and particularly with older filesystems that have had a lot of use before some of the recent bugfixes, that's not always a given, and recovery (or restore from backups, which given btrfs experimental status, are even MORE important than they'd be on a filesystem considered stable, basically, consider all your data on btrfs as "throw away" if it comes to it -- keep your primary copy as well as its backups on something other than btrfs at least until that experimental label comes off) is occasionally necessary. > Another naive question: if I shutdown the system between checkpoints, > systemd should umount my partitions. Does the syncing of cached data > occur after the graceful umount? As is normally the case on Linux, once the graceful umounts (or remount- read-onlys) have fully happened, you should be good to go. Syncing should be completed before the umount (or remount read-only) is completed, so you should be safe after that. There have however been a few bugs, to my knowledge now all fixed, where the umount wouldn't complete (livelock), and even a few where it would appear to complete but the filesystem was continuing to do stuff in the background, such that shutting it down before that was complete would result in corruption. AFAIK a shutdown after initiating a btrfs balance, before it completed, used to be one such situation, but btrfs now properly suspends the balance and quiesces the filesystem before umount, resuming it at remount read/write. On a slow multi-terabyte "spinning rust" filesystem, a full balance can take quite some time (hours to tens of hours), so not being able to properly gracefully suspend that balance for umount and resume after a remount was a BIG problem, now fixed, as I said. But the caveat about btrfs' experimental/developmental status remains -- there ARE still bugs being found and fixed; choose a fully stable filesystem, not the still experimental btrfs, if you're not willing to keep backups and consider everything you put on btrfs for the time being subject to potential loss if the worst should happen. And of course as the wiki[1] recommends, if you do choose to run btrfs, keep current on your kernels, as they really ARE fixing bugs in real- time, and if you're running a kernel older than the latest Linus stable series, you *ARE* going to be missing bugfixes that just /might/ save you from serious btrfs problems. --- [1] Btrfs wiki: https://btrfs.wiki.kernel.org/ -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
