Re: [PATCH v3 07/14] btrfs-progs: Doc/mkfs: Add extra condition for rootdir option

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2017-09-22 11:07, Qu Wenruo wrote:
> 
> 
> On 2017年09月22日 21:33, Austin S. Hemmelgarn wrote:
>> On 2017-09-22 08:32, Qu Wenruo wrote:
>>> On 2017年09月22日 19:38, Austin S. Hemmelgarn wrote:
>>>> On 2017-09-22 06:39, Qu Wenruo wrote:
>>>>> As I already stated in an other thread, if you want to shrink, do 
>>>>> it in another command line tool.
>>>>> Do one thing and do it simple. (Although Btrfs itself is already 
>>>>> out of the UNIX way)
>>>> Unless I'm reading the code wrong, the shrinking isn't happening in 
>>>> a second pass, so this _is_ doing one thing, and it appears to be 
>>>> doing it as simply as possible (although arguably not correctly 
>>>> because of the 1MB reserved area being used).
>>>
>>> If you're referring to my V1 implementation of shrink, that's doing 
>>> *one* thing.
>>>
>>> But the original shrinking code? Nope, or we won't have the custom 
>>> chunk allocator at all.
>>>
>>> What I really mean is, if one wants to shrink, at least don't couple 
>>> the shrink code into "mkfs.btrfs".
>>>
>>> Do shrink in its own tool/subcommand, not in a really unrelated tool.
>> There are two cases for shrinking a filesystem:
>> 1. You're resizing it to move to a smaller disk (or speed up copying 
>> to another disk).
>> 2. You're generating a filesystem image that needs to be as small as 
>> possible.
> 
> I would argue there is no meaning of creating *smallest* image. (Of 
> course it exists).
There is an exact meaning given the on-disk layout.  It's an image whose 
size is equal to the sum of:
1. 1MB (for the reserved space at the beginning).
2. However many superblocks it should have given the size.
3. The total amount of file data and extended attribute data to be 
included, rounding up for block size
4. The exact amount of metadata space needed to represent the tree from 
3, also rounding up for block size.
5. The exact amount of system chunk space needed to handle 3 and 4, plus 
enough room to allocate at least one more chunk of each type (to 
ultimately allow for resizing the filesystem if desired).
6. Exactly enough reserved metadata space to resize the FS.
> 
> We could put tons of code to implement, and more (or less) test cases to 
> verify it.
> 
> But the demand doesn't validate the effort.
And how much effort has been put into ripping this out completely 
together with the other fixes?  How much more would it have been to just 
move it to another option and fix the reserved area usage?
> 
> All my points are clear for this patchset:
> I know I removed one function, and my reason is:
> 1) No or little usage
>     And it's anti intuition.
So split it to a separate tool (mkimage maybe?), and fix mkfs to behave 
sensibly.  I absolutely agree on the fact that it's non-intuitive.  It 
should either be it's own option (with a dependency on -r being passed 
of course), or a separate tool if you're so worried about mkfs being too 
complex.

As to usage, given the current data, there is no proof that I'm the only 
one using it, but there is also no proof that anybody other than me is 
using it, which means that you can't reasonably base an argument on 
actual usage of this option, since you can't prove anything about usage. 
  All you know is that you have one person who uses it, and one who was 
confused by it (but appears to want to use it in a different way).  It's 
a niche use case though, and when dealing with something like this, 
there is a threshold of usage below which you won't see much in the way 
of discussion of the option on the list, since only a reasonably small 
percentage of BTRFS users are actually subscribed.

> 2) Dead code (not tested nor well documented)
<rant>It _IS NOT_ dead code.  It is absolutely reachable from code 
external to itself.  It's potentially unused code, but that is not the 
same thing as dead code.</rant>

That aside, I can fix up the documentation, and I've actually tested it 
reasonably thoroughly (I use it every month or so when I update stuff I 
have using seed devices, and it also gets used by my testing 
infrastructure when generating images pre-loaded with files for tests to 
save time).  I'll agree it hasn't been rigorously tested, but it does 
appear to work as (not) advertised, even when used in odd ways.

> 3) Possible workaround
There are three options for workarounds, and both of them are sub-par to 
this even aside from the reduced simplicity it offers to userspace:
1. Resize after mkfs.  This is impractical both because there is no 
offline resize (having to mount the FS RW prior to use as a seed device 
means that you don't have a guaranteed reproducible image, which is a 
pretty common request for container usage there days), and it will end 
up with wasted space (the smallest possible filesystem created through a 
resize is consistently larger (by more than 1MB) than what the -r option 
to mkfs generates).
2. Use a binary search to determine the smallest size to a reasonable 
margin.  This is impractical simply because it takes too long, and again 
can't reliably get the smallest possible image.
3. Attempt to compute the smallest possible image without using a binary 
search, pre-create the file, and then call mkfs.  This is non-trivial 
without knowledge of the internal workings of mkfs, and is liable to 
break when something changes in mkfs (unless you want to consider the 
block-level layout generated by the --rootdir option to be part of the 
ABI and something that shouldn't change, but that is something you would 
need to discuss with the other developers).

IOW, this is like saying that duct tape is a workaround for not having 
super glue.  It will technically work, but not anywhere near as well.
> 
> I can add several extra reasons as I stated before, but number of 
> reasons won't validate anything anyway.
> 
> Building software is always trading one thing for another.
> I understand there may be some need for this function, but it doesn't 
> validate the cost.
> 
> And I think the fact that until recently a mail reported about the 
> shrinking behavior has already backed up my point.
The only information it gives is that until now nobody who tried that 
option either cared enough to complain about it, or needed it to behave 
any other way.

IOW, as stated above, given the current data, there is no proof that I'm 
the only one using it, but there is also no proof that anybody other 
than me is using it, which means that you can't reasonably base your 
argument on actual usage of this option.
> 
> Thanks,
> Qu
> 
>>
>> Case 1 is obviously unrelated to creating a filesystem.  Case 2 
>> however is kind of integral to the creation of the filesystem image 
>> itself by definition, especially for a CoW filesystem because it's not 
>> possible to shrink to the absolute smallest size due to the 
>> GlobalReserve and other things.
>>
>> Similarly, there are two primary use cases for pre-loading the 
>> filesystem with data:
>> 1. Avoiding a copy when reprovisioning storage on a system.  For 
>> example, splitting a directory out to a new filesystem, you could use 
>> the -r option to avoid having to copy the data after mounting the 
>> filesystem.
>> 2. Creating base images for systems.
>>
>> The first case shouldn't need the shrinking functionality, but the 
>> second is a very common use case together with the second usage for 
>> shrinking a filesystem.
>>>
>>>>>
>>>>> It may be offline shrink/balance. But not to further complexing the 
>>>>> --rootdir option now. >
>>>>> And you also said that, the shrink feature is not a popular feature 
>>>>> *NOW*, then I don't think it's worthy to implment it *NOW* either.
>>>>> Implement future feature in the future please.
>>>> I'm not sure about you, but I could have sworn that he meant seed 
>>>> devices weren't a popular feature right now,
>>>
>>> Oh, sorry for my misunderstanding.
>>>
>>>> not that the shrinking is. As a general rule, the whole option of 
>>>> pre-loading a filesystem with data as you're creating it is not a 
>>>> popular feature, because most sysadmins are much more willing to 
>>>> trust adding data after the filesystem is created.
>>>>
>>>> Personally, given the existence of seed devices, I would absolutely 
>>>> expect there to be a quick and easy way to generate a minimalistic 
>>>> image using a single command (because realistic usage of seed 
>>>> devices implies minimalistic images).  I agree that it shouldn't be 
>>>> the default behavior, but I don't think it needs to be removed 
>>>> completely.
>>>
>>> Just like I said in cover letter, even for ext*, it's provided by 
>>> genext2fs, not mke2fs.
>> Then maybe this should get split out into a separate tool instead of 
>> just removing it completely?  There is obviously at least some 
>> interest in this functionality.
>>>
>>> I totally understand end-user really want a do-it-all solution.
>>> But from developers' view, the old UNIX way is better to maintain 
>>> code clean and easy to read.
>> What the code is doing should have near zero impact on readability.  
>> If it did, then the BTRFS code in general is already way beyond most 
>> people.
>>>
>>>
>>> In fact, you can even create your script to do the old behavior if 
>>> you don't care that the result may not fully take use of the space, 
>>> just by:
>>>
>>> 1) Calculate the size of the whole directory
>>>     "du" command can do it easily, and it does things better than us! 
>>> For
>>>     years!
>> Um, no it actually doesn't do things better in all cases.  it doesn't 
>> account for extended attributes, or metadata usage, or any number of 
>> other things that factor into how much space a file or directory will 
>> take up on BTRFS.  It's good enough for finding what's using most of 
>> your space, but it's not reliable for determining how much space you 
>> need to store that data (especially once you throw in in-line 
>> compression).
>>>
>>> 2) Multiple the value according to the meta/data profile
>>>     Take care of small files, which will be inlined.
>>>     And don't forget size for data checksum.
>>>     (BTW, there is no way to change the behavor of inlined data and data
>>>      checksum for mkfs. unlike btrfs-convert)
>> This is where the issue lies.  It's not possible for a person to 
>> calculate this with reasonable accuracy, and you arguably can't even 
>> do it for certain programmatically without some serious work.
>>>
>>> 3) Create a file with size calculated by step 2)
>>>
>>> 4) Execute "mkfs.btrfs -d <dir> <created file>"
>>>
>>>>   The main issues here are that it wasn't documented well (like many 
>>>> other things in BTRFS), and it didn't generate a filesystem that was 
>>>> properly compliant with the on-disk format (because it used space in 
>>>> the 1MB reserved area at the beginning of the FS).  Fixing those 
>>>> issues in no way requires removing the feature.
>>>
>>> Yes, 1MB can be fixed easily (although not properly). But the whole 
>>> customized chunk allocator is the real problem.
>>> The almost dead code is always bug-prone. Replace it with updated 
>>> generic chunk allocator is the way to avoid later whac-a-mole, and 
>>> should be done asap.
>> Agreed, but that doesn't preclude having the option to keep the 
>> generated image to the minimum size.
>>>
>>>>>
>>>>> And further more, even following the existing shrink behavior, you 
>>>>> still need to truncate the file all by yourself.
>>>>> Which is no better than creating a good sized file and then mkfs on 
>>>>> it.
>>>> Only if you pre-create the file.  If the file doesn't exist, it gets 
>>>> created at the appropriate size.  That's part of why the chunk 
>>>> allocations are screwed up and stuff gets put in the first 1MB, it 
>>>> generates the FS on-the-fly and writes it out as it's generating it.
>>>
>>> Nope, even you created the file in advance, it will still occupy the 
>>> first 1M.
>> Because it doesn't assume that the file is there to begin with.  It's 
>> not trying O_CREAT and falling back to some different code if that 
>> fails.  The code assumes that the file won't be there, and handles 
>> things accordingly albeit incorrectly (it should seek past the first 
>> 1MB, write the initial SB, and then start chunk allocation).  IOW, the 
>> code takes a shortcut in that it doesn't check for the file, and the 
>> rest is written to account for that by assuming there wasn't a file. 
>> The lack of truncation just means it doesn't try to trim things down 
>> by itself if the file is already there (it assumes that you knew what 
>> you were doing).
>>
>> Put differently, I'm fairly certain that the current -r option removes 
>> the total size check unless the target is a device (although it may 
>> remove the check there too and just fail when it tries to write past 
>> the end of the device), and will thus extend existing files to the 
>> required size to hold the data.
>>>
>>> BTW, you can get back the size calculation for shrink, but you will 
>>> soon find that it's just the start of a new nightmare.
>>> Because there is no easy way to calculate the real metadata usage.
>>>
>>> The result (and the old calculator) will be no better than guessing it.
>>> (Well, just multiply the dir size by 2 will never go wrong)
>> No, it can go wrong depending on what you count as part of the size.
>>>
>>>
>>> Thanks,
>>> Qu
>>>
>>>>>
>>>>> Thanks,
>>>>> Qu
>>>>>
>>>>> Sent: Friday, September 22, 2017 at 5:24 PM
>>>>> From: "Anand Jain" <anand.jain@oracle.com>
>>>>> To: "Qu Wenruo" <quwenruo.btrfs@gmx.com>, linux-btrfs@vger.kernel.org
>>>>> Cc: dsterba@suse.cz
>>>>> Subject: Re: [PATCH v3 07/14] btrfs-progs: Doc/mkfs: Add extra 
>>>>> condition for rootdir option
>>>>>
>>>>>> +WARNING: Before v4.14 btrfs-progs, *--rootdir* will shrink the 
>>>>>> filesystem,
>>>>>> +prevent user to make use of the remaining space.
>>>>>> +In v4.14 btrfs-progs, this behavior is changed, and will not 
>>>>>> shrink the fs.
>>>>>> +The result should be the same as `mkfs`, `mount` and then `cp -r`. +
>>>>>
>>>>> Hmm well. Shrink to fit exactly to the size of the given
>>>>> files-and-directory is indeed a nice feature. Which would help to 
>>>>> create
>>>>> a golden-image btrfs seed device. Its not popular as of now, but at 
>>>>> some
>>>>> point it may in the cloud environment.
>>>>>
>>>>> Replacing this feature instead of creating a new option is not a good
>>>>> idea indeed. I missed something ?
>>>>>
>>>>> Thanks, Anand
>>>>>
>>>>>> +Also, if destination file/block device does not exist, 
>>>>>> *--rootdir* will not
>>>>>> +create the image file, to make it follow the normal mkfs behavior.
>>>>
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