在 2020年10月28日 16:32, HAGIO KAZUHITO(萩尾 一仁) 写道:
> Hi Julien,
>
> sorry for my delayed reply.
>
> -Original Message-
>>> A user might want to know how much space a vmcore file will take on
>>> the system and how much space on their disk should be available to
>>> save it during a crash.
>>>
>>> The option --vmcore-size does not create the vmcore file but provides
>>> an estimation of the size of the final vmcore file created with the
>>> same make dumpfile options.
>>>
>>> Interesting. Do you have any actual use case? e.g. used by kdumpctl?
>>> or use it in kdump initramfs?
>>>
>>
>> Yes, the idea would be to use this in mkdumprd to have a more accurate
>> estimate of the dump size (currently it cannot take compression into
>> account and warns about potential lack of space, considering the system
>> memory size as a whole).
>
> Hmm, I'm not sure how you are going to implement in mkdumprd, but I do not
> recommend that you use it to determine how much disk space should be
> allocated for crash dump. Because, I think that
>
> - It cannot estimate the dump size when a real crash occurs, e.g. if slab
> explodes with non-zero data, almost all memory will be captured by
> makedumpfile
I agree with you, but this could be rare? If yes, I'm not sure if it is worth
thinking more about the rare situations.
> even with -d 31, and compression ratio varies with data in memory.
Indeed.
> Also, in most cases, mkdumprd runs at boot time or construction phase
> with less memory usage, not at usual application running time. So it
> can underestimate the needed size easily.
>
If administrator can monitor the estimated size periodically, maybe it
won't be a problem?
> - The system might need a full vmcore and need to change makedumpfile's
> dump level for an issue in the future. But many systems cannot change
> their disk space allocation easily. So we should prevent users from
> having minimum disk space for crash dump.
>
> So, the following is from mkdumprd on Fedora 32, personally I think this
> is good for now.
>
> if [ $avail -lt $memtotal ]; then
> echo "Warning: There might not be enough space to save a vmcore."
> echo " The size of $2 should be greater than $memtotal kilo
> bytes."
> fi
>
Currently, some users are complaining that mkdumprd overestimates the needed
size,
and most vmcores are significantly smaller than the size of system memory.
Furthermore, in most cases, the system memory will not be completely exhausted,
but
that still depends on how the memory is used in the system, for example:
[1] make the stressful test for memory
[2] always occupies amount of memory and not release it.
For the above two cases, there may be rare. Therefore, can we find out a
compromise
between the size of vmcore and system memory so that makedumpfile can estimate
the
size of vmcore more accurately?
And finally, mkdumprd can use the estimated size of vmcore instead of system
memory(memtotal)
to determine if the target disk has enough space to store vmcore.
Thanks.
Lianbo
> The patch's functionality itself might be useful and I don't reject, though.
>
>>> @@ -4643,6 +4706,8 @@ write_buffer(int fd, off_t offset, void *buf,
>>> size_t buf_size, char *file_name)
>>> }
>>> if (!write_and_check_space(fd, , sizeof(fdh),
>>> file_name))
>>> return FALSE;
>>> + } else if (info->flag_vmcore_size && fd == info->fd_dumpfile) {
>>> + return write_buffer_update_size_info(offset, buf,
>>> buf_size);
>>>
>>> Why do we need this function? makedumpfile actually writes zero-filled
>>> pages to the dumpfile with -d 0, and doesn't write them with -d 1.
>>> So isn't "write_bytes += buf_size" enough? For example, with -d 30,
>>>
>>
>> The reason I went with this method was to make an estimate of the number
>> of blocks actually allocated on the disk (since depending on how the
>> data written is scattered in the file, there might be a significant
>> difference between bytes written vs actual size allocated on disk). But
>> I realize that there is some misunderstanding from my end since written
>> 0 do make block allocation as opposed to not writing at some offset
>> (skipping the with lseek() ), I would need to fix that.
>>
>> To highlight the behaviour I'm talking about:
>> $ dd if=/dev/zero of=./testfile bs=4096 count=1 seek=1
>> 1+0 records in
>> 1+0 records out
>> 4096 bytes (4.1 kB, 4.0 KiB) copied, 0.000302719 s, 13.5 MB/s
>> $ du -h testfile
>> 4.0K testfile
>>
>> $ dd if=/dev/zero of=./testfile bs=4096 count=2
>> 2+0 records in
>> 2+0 records out
>> 8192 bytes (8.2 kB, 8.0 KiB) copied, 0.000373002 s, 22.0 MB/s
>> $ du -h testfile
>> 8.0K testfile
>>
>>
>> So, do you think it's not worth bothering estimating the number of
>> blocks allocated an that I should only consider the number of bytes written?
>
> Yes, makedumpfile
