It would be interesting to see comparison between ZFS and Btrfs, ReiserFS in terms of features and development effort. Any idea?
btrfs is a shitshow - sorry to having to resort to such words but I will refrain from using it ever again if I have a choice.
If you look at the btrfs wiki there a lot of features but most of these are horrible broken or implemented in a way that's surprising.
It's full of bugs and not-yet-implemented stuff - some stuff that comes to mind:
* RAID is utterly broken: - http://www.spinics.net/lists/linux-btrfs/msg48561.html - http://www.spinics.net/lists/linux-btrfs/msg47845.html
* Lot's of ENOSPC bugs (just look at the mailing list)
* You need to run the latest kernel (don't try anything below 4.2) - if not: Data corruption bugs, deadlocks
* Quota is utterly broken and continues to be breaking (just look at the mailinglist)
* It's useless for databases or VM images (disabling datacow is a ugly hack IMHO) - e.g. see here: http://blog.pgaddict.com/posts/friends-dont-let-friends-use-...
ZFS is not perfect either especially on Linux it's lacking tight integration into the VM subsystem and at least a few versions ago was prone to deadlocking, too. However it's not eating data and compared to btrfs it's rock solid.
sigh From the message that you linked:
"FWIW... Older btrfs userspace such as your v3.17 is "OK" for normal runtime use, assuming you don't need any newer features, as in normal runtime, it's the kernel code doing the real work and userspace for the most part simply makes the appropriate kernel calls to do that work.
But, once you get into a recovery situation like the one you're in now, current userspace becomes much more mportant, as the various things you'll do to attempt recovery rely far more on userspace code directly accessing the filesystem, and it's only the newest userspace code that has the latest fixes."
If you look, you see that the guy reporting the problem was using a very new kernel, but a one-year-old btrfs-tools package. It makes no sense to use tools that are substantially older than the kernel you're running. He might have had much better luck with recovery (that is, he might have been able to get a rw volume with all his data, rather than an ro volume -with all his data- out of the process) with a recent tools package.
> http://www.spinics.net/lists/linux-btrfs/msg47845.html
Maybe I'm reading this one wrong, but it looks like the summary of the issues are:
* For multi-disk arrays with disks of varying sizes, each smaller disk does not receive data until all larger disks have as much or less free space than the smaller disk in question. [0]
* The read scheduling algo is currently naive, using PID evenness to determine the disk in a pair to read, rather than current IO load on the disk.
Did I miss something, or misunderstand something? If I didn't miss anything, I don't see how you can get "RAID is utterly broken" from "RAID has less-than-optimal write and read scheduling".
> * It's useless for databases or VM images (disabling datacow is a ugly hack IMHO)
I can't agree. I use btrfs on spinning rust to store my multi-terabyte Postgres 9.4 DB. It works well enough for my low-to-medium-volume [1] workload.
> * Lot's of ENOSPC bugs (just look at the mailing list)
I don't run into those, and haven't for literal years.
> * Quota is utterly broken...
Yes, quota is a difficult problem that continues to be unsolved.
> * You need to run the latest kernel...
Yes. Agreed.
> ...if not: Data corruption bugs ...
I haven't run into any, ever on the handful of systems on which I use BTRFS.
> ...if not: ... deadlocks
True. There was that unlink/move deadlock in 4.1. I hit it from time to time when updating my copy of the metadata from the Gentoo Portage tree. Happily, it only blocked operation on the affected file. The remainder of the system kept running along, and a -ugh- reboot unblocked the operation. [2]
Look. I'm not saying that btrfs is feature-complete, suitable for every workload, or that any given person should use it for their workload. I'm just saying that the situation is far less dire than you're making it out to be. I've been using it as the rootfs for at least one of my daily use systems for the past 5.5 years, and haven't had any trouble out of it [3] in the past 2+ years.
[0] That phrasing was abysmal. Here's an example: Given an array of one each of 3GB, 2GB, 1.5GB devices. the 2GB device will get data after 1GB has been written to the 3GB device. The 1.5GB device will get data after 0.5GB of additional data has been written to each of the 3GB and 2GB devices (making a total of 1GB additional data written to the array before the 1.5GB device starts getting data). AIUI, if you're running in RAID1 mode, then the same logic applies, but with paired devices.
[1] Most definitely not web-scale workload.
[2] I don't know if a unmount/remount cycle would have also worked. The wedged operation happened on my rootfs, so I had no way to test this.
[3] Except for the unlink/move deadlock in 4.1.
[RAID]
>sigh From the message that you linked:
The part below your quote is interesting.
> General note about btrfs and btrfs raid. Given that btrfs itself remains a "stabilizing, but not yet fully mature and stable filesystem", while btrfs raid will often let you recover from a bad device, sometimes that recovery is in the form of letting you mount ro, so you can access the data and copy it elsewhere, before blowing away the filesystem and starting over.
I have hotplug and online-replace with ZFS and mdraid. btrfs wants a special degraded remount that might or might not be able to recover to rw someday... replacing a disk online is not really possible. Handling a failing drive is not really implemented.
Don't call it stable or don't call it RAID. It's nice that I'm able to pull of the data of my ro-mounted degraded array when a disk fails.. however all of this stuff is not really documented and was very surprising to me. If you have production machine e.g. on btrfs RAID10 it sucks to not be able to replace a broken drive on the fly online.
ZFS/mdraid just do it..
[RAID II]
> * The read scheduling algo is currently naive, using PID evenness to determine the disk in a pair to read, rather than current IO load on the disk.
> Did I miss something, or misunderstand something? If I didn't miss anything, I don't see how you can get "RAID is utterly broken" from "RAID has less-than-optimal write and read scheduling".
Walking is a less than than optimal way to move from Moscow to Berlin. If I take a train I expect a train - btrfs is a train that occasionally stops and says: Well, just walk... okay, you see I'm kind of emotional invested in this shit. However it's not what's usually expected as RAID and it's not documented.
We have here a lot of applications that do exactly that: a single PID reads lots of stuff very often sequentially... so RAID1 doubles throughput. Well not on btrfs.
[ENOSPC]
> I don't run into those, and haven't for literal years.
4x4TB RAID10 (90% free) -> ENOSPC - this was somehwere between 3.16 or 3.19 - how can this even happen?
[Corruption]
> I haven't run into any, ever on the handful of systems on which I use BTRFS.
Good for you. Was hit by this nasty undeletable directory bug: http://www.spinics.net/lists/linux-btrfs/msg35789.html
And then this: http://www.spinics.net/lists/linux-btrfs/msg45108.html
Umounting and running btrfs --check repair is not really cool if you want some uptime...
[Deadlocks]
Currently btrfs deadlocks reliable in a OOM situation... With 4.2 things got better, before running some medium heavy Hadoop job or ElastichSearch indexing killed 20% to 50% of the machines due to deadlocks (with 3.19 IIRC)
> Look. I'm not saying that btrfs is feature-complete, suitable for every workload, or that any given person should use it for their workload. I'm just saying that the situation is far less dire than you're making it out to be. I've been using it as the rootfs for at least one of my daily use systems for the past 5.5 years, and haven't had any trouble out of it [3] in the past 2+ years.
It's probably fine for personal use. But don't think about using it for something that is demanding. You can google the presentations that say: Use it! It's stable! Enterprise! It's not.
Maybe in 5 years. I just saw it breaking in too many strange ways to bother anymore with it.
No worries. I don't give a shit about tone, just about content. Tone is next to impossible to accurately judge through text anyway, so I don't presume to understand someone's intent unless I have a pile of evidence. :-)
> ...btrfs wants a special degraded remount that might or might not be able to recover to rw someday... replacing a disk online is not really possible.
But, from the email you quoted:
"...while btrfs raid will often let you recover from a bad device, sometimes that recovery is in the form of letting you mount ro, so you can access the data and copy it elsewhere, before blowing away the filesystem and starting over."
And [0] indicates that you can live-replace a failed drive in a btrfs RAID array. (The man page makes it seem that you might want to also pass the -r flag to the replace operation.)
I've never had the money around to amass the disks required to run my btrfs array in RAID1 data mode, so I can't test the claim of that Stack Overflow post, but I do use it in single data, RAID1 metadata and system mode. I've had a drive in that array suddenly drop out due to a firewire controller hiccup, and I suffered no data loss. (The volume did get forced into ro mode, so future writes were lost, but all applications knew that those writes were lost.)
Additionally, I've been able to live-add and live-remove drives from my array with no hassle (other than the expected speed degradation due to shuffling the data around) whatsoever.
> ...so RAID1 doubles [read] throughput. Well not on btrfs.
Maybe I'm out of the loop, but I've never heard anyone say "use RAID1 to double your read throughput". That sounds like a very nice thing to have, but far from essential. Its absence certainly doesn't make btrfs's RAID1 implementation broken. :)
> Umounting and running btrfs --check repair is not really cool if you want some uptime...
Agreed, very much so.
One thing bothers me about that guy who was reporting the bug on the 4.1 kernel. He claimed that his mount options were:
/dev/sdb /media/storage2 btrfs ro,noatime,compress=lzo,space_cache 0 0
It's also interesting that noone replied to his messages. Both linux-btrfs and #btrfs have always been quite responsive when I've sent messages.
> 4x4TB RAID10 (90% free) -> ENOSPC - this was somehwere between 3.16 or 3.19 - how can this even happen?
Dunno. It looks like the Global Reserve got introduced in ~3.16, so it's not likely related to that. Maybe you hit a hard-link limit or had really, really large xattrs attached to files or something? (I do know that btrfs has (had?) a surprisingly low limit on the number of hard links supported in a volume, but I have no idea how hitting that limit presents itself.)
Do you remember if you let the system idle for a few minutes and tried your operation again? (Not that that's a reliable [or even reasonable] solution, mind, but does provide a useful piece of diagnostic info.)
In the distant past, running a balance on empty chunks (in order to free them and free up otherwise unused metadata space) was one recommended way to work around mysterious ENOSPC issues. This isn't supposed to be required anymore, but it's a last-ditch cargo-cult thing that can't do anything worse than waste one's time.
> Good for you. Was hit by this nasty undeletable directory bug...
That seriously sucks. However, I don't call that data corruption. Inconvenient, bordering on unacceptable? Yes. Data corruption? Nah. Your data is still present and correct, but you can't get rid of some directories. [1]
> Currently btrfs deadlocks reliable in a OOM situation... With 4.2 things got better...
Is it any OOM situation, or just particular ones? I know that I've had google-chrome + Hangouts run away and nom all my memory on multiple occasions, and had my btrfs volumes function just fine after the OOM killer had its way with Chrome.
Also, do you have a handle on roughly how much better it has become in 4.2, and is this problem something that the btrfs folks are aware of?
> [btrfs is] probably fine for personal use. But don't think about using it for something that is demanding.
That's a far softer statement than "btrfs is a shitshow ... I will refrain from using it ever again if I have a choice.". :)
Anyway, I do REALLY hope I soon get the budget to set up a btrfs testbed. I'd really like to see (and help diagnose) all these failures that people keep talking about... in a carefully controlled environment, with data that I don't care about -naturally-. :)
[0] http://superuser.com/questions/685364/can-a-failed-btrfs-dri...
[1] And -for the record- I saw an eerily similar thing happen every other month at a Windows-only programming shop I used to work for. One or more files and/or directories on the NTFS-backed Windows 2003 file server would inevitably become undeletable. [2] The sysadmin would have to take the whole thing down for a half hour to smear chicken guts in the right places to get rid of the offending files/directories. Despite these regular issues, we never decided to stop using NTFS. ;)
[2] And no, noone who was using the file server had admin access. Unprivved user access was all that was needed to create these undeletable files and directories.
Good catch! I have to test on a rented server I'm running anyway in the next days - adding a an additional drive, running replace and removing the old drive seems to work - I'm about to find out if - deleting a device, rebooting and trying to replace an non-existent device also works - I guess it's likely that I have to fumble with the provider-rescue system (so now control about the kernel version :/)
I thought that the -o degraded dance was still necassary, looks like I was wrong. Let's see how the disk replacement on that servers turns out...
> One thing bothers me about that guy who was reporting the bug on the 4.1 kernel. He claimed that his mount options were: /dev/sdb /media/storage2 btrfs ro,noatime,compress=lzo,space_cache 0 0 but the issue he was reporting smelled like it could only happen on a rw volume. Wonder what was up with that.
That guy was me :)
Basically the disk got remounted ro after the failure and I just did a grep on /proc/mounts for the fs before posting (not very wise). This has been a bug that was fixed with 4.2 (didn't saw it again at least) and btrfs check --repair fixed it.
> It's also interesting that noone replied to his messages. Both linux-btrfs and #btrfs have always been quite responsive when I've sent messages.
I had no time to hang around IRC at this time. Yes. Usually it's very nice there. I also want to say I'm not ranting against any of the people involved. I'm just unhappy with the current state of the technology.
[ENOSPC] > Dunno. It looks like the Global Reserve got introduced in ~3.16, so it's not likely related to that. Maybe you hit a hard-link limit or had really, really large xattrs attached to files or something? (I do know that btrfs has (had?) a surprisingly low limit on the number of hard links supported in a volume, but I have no idea how hitting that limit presents itself.)
Actually I don't know - the filesystem in question ran a while with the 3.13 kernel and got later upgraded to something less old. Maybe that's part of the problem. ENOSPC happened while a misconfigured Hadoop NameNode wrote tons of data 4x without checkpointing on the disk and when restarting the checkpointing server it all crashed. It's now running 4.2 and fine so far...
> In the distant past, running a balance on empty chunks (in order to free them and free up otherwise unused metadata space) was one recommended way to work around mysterious ENOSPC issues. This isn't supposed to be required anymore, but it's a last-ditch cargo-cult thing that can't do anything worse than waste one's time.
Yeah did that balance dance and it somewhat worked.
> Is it any OOM situation, or just particular ones? I know that I've had google-chrome + Hangouts run away and nom all my memory on multiple occasions, and had my btrfs volumes function just fine after the OOM killer had its way with Chrome.
Never got around to nail down this one (would love to being able to file a bug on this) - but at the time I was not really able to do much on the machines - it's a NUMA system and basically ElasticSearch and Hadoop allocated far more than 64GB memory - OOM kicked in and _after a while_ the logs where full of hanging tasks related to btrfs - at that time I've found something related on the mailing list in regards to allocations and OOM however I'm lacking the intimate knowledge to be sure if it's related.
I've screamed at everyone to avoid OOM situations so I'm not sure if it's still a problem :)
> Also, do you have a handle on roughly how much better it has become in 4.2, and is this problem something that the btrfs folks are aware of?
So far I did not see it again. I had not enough data to file a bug or investigate further. It was something lock related IIRC.
> Anyway, I do REALLY hope I soon get the budget to set up a btrfs testbed. I'd really like to see (and help diagnose) all these failures that people keep talking about... in a carefully controlled environment, with data that I don't care about -naturally-. :)
Good luck! You can try running Hadoop and some heavy jobs on it. Lot's of threads that read and write lot's of data. There are some benchmarks like https://github.com/intel-hadoop/HiBench to stress the system. Should also work on a single system.
> [1] And -for the record- I saw an eerily similar thing happen every other month at a Windows-only programming shop I used to work for. One or more files and/or directories on the NTFS-backed Windows 2003 file server would inevitably become undeletable. [2] The sysadmin would have to take the whole thing down for a half hour to smear chicken guts in the right places to get rid of the offending files/directories. Despite these regular issues, we never decided to stop using NTFS. ;)
Yes. There is worse stuff out there. I was using ZFS before with lots of really fucked up disks in a kind of "don't do this, you are stupid!" setup and ZFS just did it's job and never complained, same Hadoop workload + disks with thousands of bad sectors. Maybe it's unfair to compare btrfs to this, however it was a ride full of disillusionment to think btrfs compares to ZFS.
Maybe it's getting there and I was in at a bad time... I'm not so sure through.
There was a talk "Why OpenBSD sucks" by Henning Brauer, where he complains that ZFS is a "kitchen sink" approach to a filesystem. He doesn't really explain further, but it would be interesting to hear someone elaborate that criticism.
Keep in mind that their criticisms are mostly about it not being a good choice for OpenBSD, although some of them apply across the board.
[0] http://www.tedunangst.com/flak/post/ZFS-on-OpenBSD
[1] http://www.bsdnow.tv/episodes/2014_02_05-time_signatures
Thanks for info about Henning Brauer talk, will check it out.
It is very easy to crank out claims of features while not having really implemented said feature in a correct way. This leads to bugs which are hard to find and almost impossible to eradicate.