RE: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file?
> > > -Original Message- > From: Grant Edwards > Sent: Saturday, November 12, 2022 7:55 PM > To: gentoo-user@lists.gentoo.org > Subject: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file? > > On 2022-11-12, Michael wrote: > > > Have your questions been answered satisfactorily by Lawrence's contribution? > > Yes, Lawrence's experiment answered the my question: e2fsck adds the bad > block to the "bad block" inode and leaves it also allocated to the existing > file. > > Presumably if you don't allow it to clone the block, reading that file will > return an error when it gets to the bad block. Once you delete that file, the > bad block will never get reallocated by the filesystem since it still belongs > to the bad block inode. > > The failing SSD that prompted the question has now been replaced and a fresh > Gentoo system installed on the new drive. I never did figure out which files > contained the bad blocks (there were 37 bad blocks, IIRC). They apparently > didn't belong to any of the files I copied over to the replacement drive. > > The old drive was a Samsung 850 EVO SATA drive, and the new one is a Samsung > 980 PRO M.2 drive. The new one is noticably faster than the old one (which in > turn was way faster than the spinning platter drive it had replaced). > > -- > Grant Multiply-allocated blocks won't cause an error by themselves. They can just cause strange and unexpected munging of your data if two files are scribbling on the same patch of disk. So if you leave it allocated to both then you can use a more intelligent tool to either coax one more read out of it or potentially replace the lost data with some substitute. I'm not sure what fsck will do with a read error while cloning the block since my test "disk" wasn't actually bad. Presumably fill the bad section with nulls. LMP
Re: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file?
On Saturday, 12 November 2022 16:44:05 GMT Grant Edwards wrote: > On 2022-11-12, Michael wrote: > > On Wednesday, 9 November 2022 16:53:13 GMT Laurence Perkins wrote: > >> Badblocks doesn't ask to write anything at the end of the run. You > >> tell it whether you want a read test, a write-read test or a > >> read-write-read-replace test at the beginning. > > > > Not to labour the point, but 'e2fsck -v -c' runs a read test and at > > the end it informs me "... Updating bad block inode", even if it > > came across no read errors (0/0/0) and consequently does not prompt > > for a fs repair. > > That's _e2fsck_ thats doing the writing at the end, not badblocks. The > statement was that _badblocks_ doesn't ask to write anything at the > end of the run. Thanks for correcting me, the badblocks man page also makes this clear. Unless an output file is specified, it will only display the list of bad blocks on its standard output. It's been a while since I had to run badblocks and forgot its behaviour. Have your questions been answered satisfactorily by Lawrence's contribution? signature.asc Description: This is a digitally signed message part.
RE: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file?
Ok, so I decided to just go and test it myself. I created a 2MiB file and formatted it as ext4 and mounted it. I created a single, 100KiB file with a test pattern in this filesystem, and then unmounted it. I found the file in the raw storage with a hex editor, and computed a block offset in the middle of it. I swapped out my badblocks executable with a dummy that just spits out that block as "bad". I ran fsck.ext4 -c on the storage file. Fsck.ext4 calls badblocks, which returns that one, "bad" block. Fsck.ext4 then automatically adds that block to the badblocks inode immediately. Then it continues with its checks, and when it gets to the part where it's checking for inode issues, it informs you that there is a block claimed by both a file inode and the badblocks inode and would you like to try to clone it into separate copies for each. Presumably there's some logic in there to make sure that the new cloned copy goes to the file and not to the badblocks inode, but I can't be bothered to dig into it that far. I'll just assume that the fsck developers have at least a half a clue. Regardless, at that point the user gets to choose if they want fsck to try to fix it automatically, or if they want to use a more subtle tool to see if they can coax one more read out of the block in question. But whether they clone the file, data-recovery-tool the file, or simply delete the file and move on, that bad sector is on the bad list and won't get allocated again. Hopefully this answers the question sufficiently. LMP -Original Message- From: Grant Edwards Sent: Wednesday, November 9, 2022 4:19 PM To: gentoo-user@lists.gentoo.org Subject: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file? On 2022-11-09, Wol wrote: > On 09/11/2022 23:31, Grant Edwards wrote: >>> If I recall correctly, it will add any unreadable blocks to its >>> internal list of bad sectors, which it will then refuse to allocate >>> in the future. > > I doubt you recall correctly. The e2fsck man page states explicitly that a -c read failure will cause the block to be added to the bad block inode. You're claiming that is not what happens? > You should ONLY EVER conclude a block is bad if you can't write to it. > Remember what I said - if I read my 8TB drive from end-to-end twice, > then I should *expect* a read error ... OK... >> I'm asking what happens to the file containing the bad block. >> Perphaps nothing. The man page says the block is added to the "bad >> block inode". If that block was already allocated, is the bad block >> is now allocated to two different inodes? > > If a read fails, you SHOULD NOT do anything. Thanks, but I'm not asking what I should do. I'm not asking what the filesystem should do. I'm not asking what disk-drive controller firmware should do or does do with failed/spare blocks. I'm asking what e2fsck -c does when the bad block is already allocated to an inode. Specifically: Is the bad block removed from the inode to which it was allocated? Is the bad block left allocated to the previous inode as well as being added to the bad block inode? We've gotten lots of answers to lots of other questions, but after re-reading the thread a few times, I still haven't seen an answer to the question I asked. > If a write fails, you move the block and mark the failed block as bad. > But seeing as you've moved the block, the bad block is no longer > allocated to any file ... Are you stating e2fsck -c will removed bad block from the inode to which it was allocated before the scan? Is it replaced with a different block? Or just left as an empty "hole" that can't be read from or written to? The e2fsck man page does not state that the bad block is removed from the old inode, only that that bad block is added to the bad block inode. If a block is allocated to an inode, I would call that "allocated to a file". It's not a file that has a visible name that shows up in a directory, but it's still a file. -- Grant
Re: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file?
On 09/11/2022 23:31, Grant Edwards wrote: If I recall correctly, it will add any unreadable blocks to its internal list of bad sectors, which it will then refuse to allocate in the future. I doubt you recall correctly. You should ONLY EVER conclude a block is bad if you can't write to it. Remember what I said - if I read my 8TB drive from end-to-end twice, then I should *expect* a read error ... I'm asking what happens to the file containing the bad block. Perphaps nothing. The man page says the block is added to the "bad block inode". If that block was already allocated, is the bad block is now allocated to two different inodes? If a read fails, you SHOULD NOT do anything. If a write fails, you move the block and mark the failed block as bad. But seeing as you've moved the block, the bad block is no longer allocated to any file ... Cheers, Wol
Re: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file?
On Tue, 08 Nov 2022 12:55:51 -0500, Laurence Perkins wrote: > > > > >-Original Message- > >From: Grant Edwards > >Sent: Tuesday, November 8, 2022 6:28 AM > >To: gentoo-user@lists.gentoo.org > >Subject: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file? > > > >On 2022-11-08, Michael wrote: > >> On Tuesday, 8 November 2022 03:31:07 GMT Grant Edwards wrote: > >>> I've got an SSD that's failing, and I'd like to know what files > >>> contain bad blocks so that I don't attempt to copy them to the > >>> replacement disk. > >>> > >>> According to e2fsck(8): > >>> > >>>-c This option causes e2fsck to use badblocks(8) program to > >>> do > >>> a read-only scan of the device in order to find any bad blocks. If > >>> any bad blocks are found, they are added to the bad block inode to > >>> prevent them from being allocated to a file or directory. If this > >>> option is specified twice, then the bad block scan will be done > >>> using a non-destructive read-write test. > >>> > >>> What happens when the bad block is _already_allocated_ to a file? > > > >> Previously allocated to a file and now re-allocated or not, my > >> understanding is with spinning disks the data in a bad block stays > >> there unless you've dd'ed some zeros over it. Even then read or write > >> operations could fail if the block is too far gone.[1] Some data > >> recovery applications will try to read data off a bad block in > >> different patterns to retrieve what's there. Once the bad block is > >> categorized as such it won't be used by the filesystem to write new data > >> to it again. > > > >Thanks. I guess I should have been more specific in my question. > > > >What does e2fsck -c do to the filesystem structure when it discovers a bad > >block that is already allocated to an existing inode? > > > >Is the inode's chain of block groups left as is -- still containing the bad > >block that (according to the man page) "has been added to the bad block > >inode"? Presumably not, since a block can't be allocated to two different > >inodes. > > > >Is the "broken" file split into two chunks (before/after the bad > >block) and moved to the lost-and-found? > > > >Is the man page's description only correct when the bad block is currently > >unallocated? > > > >-- > >Grant > > If I recall correctly, it will add any unreadable blocks to its internal list > of bad sectors, which it will then refuse to allocate in the future. > > I don't believe it will attempt to move the file to elsewhere until it is > written since: > A) what would you then put in that block? You don't know the contents. > B) Moving the file around would make attempts to recover the data from that > bad sector significantly more difficult. > > This is, however, very unlikely to come up on a modern disk since most of > them automatically remap failed sectors at the hardware level (also on write, > for the same reasons). So the only time it would matter is if you have a > disk that's more than about 20 years old, or one that's used up all its spare > sectors... > > Unless, of course, you're resurrecting the old trick of marking a section of > the disk as "bad" so the FS won't touch it, and then using it for raw data of > some kind... > > You can, of course, test it yourself to be certain with a loopback file and a > fake "badblocks" that just outputs your chosen list of bad sectors and then > see if any of the data moves. I'd say like a 2MB filesystem and write a file > full of 00DEADBEEF, then make a copy, blacklist some sectors, and hit it with > your favorite binary diff command and see what moved. This is probably > recommended since there could be differences between the behaviour of > different versions of e2fsck. Maybe its time for spinwrite -- new version coming out soon, but it might save your bacon. -- Your life is like a penny. You're going to lose it. The question is: How do you spend it? John Covici wb2una cov...@ccs.covici.com
Re: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file?
On Tuesday, 8 November 2022 17:55:51 GMT Laurence Perkins wrote: > >-Original Message- > >From: Grant Edwards > >Sent: Tuesday, November 8, 2022 6:28 AM > >To: gentoo-user@lists.gentoo.org > >Subject: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file? > > > >On 2022-11-08, Michael wrote: > >> On Tuesday, 8 November 2022 03:31:07 GMT Grant Edwards wrote: > >>> I've got an SSD that's failing, and I'd like to know what files > >>> contain bad blocks so that I don't attempt to copy them to the > >>> replacement disk. > >>> > >>> According to e2fsck(8): > >>>-c This option causes e2fsck to use badblocks(8) program to > >>> do > >>> > >>> a read-only scan of the device in order to find any bad blocks. If > >>> > >>> any bad blocks are found, they are added to the bad block inode to > >>> prevent them from being allocated to a file or directory. If this > >>> option is specified twice, then the bad block scan will be done > >>> using a non-destructive read-write test. > >>> > >>> What happens when the bad block is _already_allocated_ to a file? > >> > >> Previously allocated to a file and now re-allocated or not, my > >> understanding is with spinning disks the data in a bad block stays > >> there unless you've dd'ed some zeros over it. Even then read or write > >> operations could fail if the block is too far gone.[1] Some data > >> recovery applications will try to read data off a bad block in > >> different patterns to retrieve what's there. Once the bad block is > >> categorized as such it won't be used by the filesystem to write new data > >> to it again.> > >Thanks. I guess I should have been more specific in my question. > > > >What does e2fsck -c do to the filesystem structure when it discovers a bad > >block that is already allocated to an existing inode? > > > >Is the inode's chain of block groups left as is -- still containing the bad > >block that (according to the man page) "has been added to the bad block > >inode"? Presumably not, since a block can't be allocated to two different > >inodes. > > > >Is the "broken" file split into two chunks (before/after the bad > >block) and moved to the lost-and-found? > > > >Is the man page's description only correct when the bad block is currently > >unallocated? > > > >-- > >Grant > > If I recall correctly, it will add any unreadable blocks to its internal > list of bad sectors, which it will then refuse to allocate in the future. > > I don't believe it will attempt to move the file to elsewhere until it is > written since: A) what would you then put in that block? You don't know > the contents. B) Moving the file around would make attempts to recover the > data from that bad sector significantly more difficult. As far as I know trying to write raw data directly to a bad block e.g. with dd or hdparm will trigger the disk's controller firmware to reallocate the data from the bad block to a spare. I always thought e2fsck won't write data in a block unless it is empty. badblocks -w will write test patterns to blocks and also trigger data reallocation on any bad blocks. badblocks -n, which corresponds to e2fsck -cc will only write to empty blocks and it may or may not trigger a firmware reallocation. I'm not sure what happens at a filesystem level, when one bad block within an extent is reallocated. The extent and the previously contiguous blocks will no longer be contiguous. Does the hardware expose some SMART data to inform the OS/fs of the reallocated block, to perform a whole extent remapping? > This is, however, very unlikely to come up on a modern disk since most of > them automatically remap failed sectors at the hardware level (also on > write, for the same reasons). So the only time it would matter is if you > have a disk that's more than about 20 years old, or one that's used up all > its spare sectors... > > Unless, of course, you're resurrecting the old trick of marking a section of > the disk as "bad" so the FS won't touch it, and then using it for raw data > of some kind... > > You can, of course, test it yourself to be certain with a loopback file and > a fake "badblocks" that just outputs your chosen list of bad sectors and > then see if any of the data moves. I'd say like a 2MB filesystem and write > a file full of 00DEADBEEF, then make a copy, blacklist some sectors, and > hit it with your favorite binary diff command and see what moved. This is > probably recommended since there could be differences between the behaviour > of different versions of e2fsck. > > LMP signature.asc Description: This is a digitally signed message part.
RE: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file?
>-Original Message- >From: Grant Edwards >Sent: Tuesday, November 8, 2022 6:28 AM >To: gentoo-user@lists.gentoo.org >Subject: [gentoo-user] Re: e2fsck -c when bad blocks are in existing file? > >On 2022-11-08, Michael wrote: >> On Tuesday, 8 November 2022 03:31:07 GMT Grant Edwards wrote: >>> I've got an SSD that's failing, and I'd like to know what files >>> contain bad blocks so that I don't attempt to copy them to the >>> replacement disk. >>> >>> According to e2fsck(8): >>> >>>-c This option causes e2fsck to use badblocks(8) program to do >>> a read-only scan of the device in order to find any bad blocks. If >>> any bad blocks are found, they are added to the bad block inode to >>> prevent them from being allocated to a file or directory. If this >>> option is specified twice, then the bad block scan will be done >>> using a non-destructive read-write test. >>> >>> What happens when the bad block is _already_allocated_ to a file? > >> Previously allocated to a file and now re-allocated or not, my >> understanding is with spinning disks the data in a bad block stays >> there unless you've dd'ed some zeros over it. Even then read or write >> operations could fail if the block is too far gone.[1] Some data >> recovery applications will try to read data off a bad block in >> different patterns to retrieve what's there. Once the bad block is >> categorized as such it won't be used by the filesystem to write new data to >> it again. > >Thanks. I guess I should have been more specific in my question. > >What does e2fsck -c do to the filesystem structure when it discovers a bad >block that is already allocated to an existing inode? > >Is the inode's chain of block groups left as is -- still containing the bad >block that (according to the man page) "has been added to the bad block >inode"? Presumably not, since a block can't be allocated to two different >inodes. > >Is the "broken" file split into two chunks (before/after the bad >block) and moved to the lost-and-found? > >Is the man page's description only correct when the bad block is currently >unallocated? > >-- >Grant If I recall correctly, it will add any unreadable blocks to its internal list of bad sectors, which it will then refuse to allocate in the future. I don't believe it will attempt to move the file to elsewhere until it is written since: A) what would you then put in that block? You don't know the contents. B) Moving the file around would make attempts to recover the data from that bad sector significantly more difficult. This is, however, very unlikely to come up on a modern disk since most of them automatically remap failed sectors at the hardware level (also on write, for the same reasons). So the only time it would matter is if you have a disk that's more than about 20 years old, or one that's used up all its spare sectors... Unless, of course, you're resurrecting the old trick of marking a section of the disk as "bad" so the FS won't touch it, and then using it for raw data of some kind... You can, of course, test it yourself to be certain with a loopback file and a fake "badblocks" that just outputs your chosen list of bad sectors and then see if any of the data moves. I'd say like a 2MB filesystem and write a file full of 00DEADBEEF, then make a copy, blacklist some sectors, and hit it with your favorite binary diff command and see what moved. This is probably recommended since there could be differences between the behaviour of different versions of e2fsck. LMP
