After unlinking a large file on ext4, the process stalls for a long time

After unlinking a large file on ext4, the process stalls for a long time

[Mason]

Hello everyone,

I'm using Linux (3.1.10 at the moment) on a embedded system similar in
spec to a desktop PC from 15 years ago (256 MB RAM, 800-MHz CPU, USB).

I need to be able to create large files (50-1000 GB) "as fast as possible".
These files are created on an external hard disk drive, connected over
Hi-Speed USB (typical throughput 30 MB/s).

Sparse files were not an acceptable solution (because the space must be
reserved, and the operation must fail if the space is unavailable).
And filling the file with zeros was too slow (typically 35 s/GB).

Someone mentioned fallocate on an ext4 partition.

So I create an ext4 partition with
$ mkfs.ext4 -m 0 -i 1024000 -O ^has_journal,^huge_file /dev/sda1
(Using e2fsprogs-1.42.10 if it matters)

And mount with "typical" mount options
$ mount -t ext4 /dev/sda1 /mnt/hdd -o noexec,noatime
/dev/sda1 on /mnt/hdd type ext4 (rw,noexec,noatime,barrier=1)

I wrote a small test program to create a large file, then immediately
unlink it.

My problem is that, while file creation is "fast enough" (4 seconds
for a 300 GB file) and unlink is "immediate", the process hangs
while it waits (I suppose) for the OS to actually complete the
operation (almost two minutes for a 300 GB file).

I also note that the (weak) CPU is pegged, so perhaps this problem
does not occur on a desktop workstation?

/tmp # time ./foo /mnt/hdd/xxx 5
posix_fallocate(fd, 0, size_in_GiB << 30): 0 [68 ms]
unlink(filename): 0 [0 ms]
0.00user 1.86system 0:01.92elapsed 97%CPU (0avgtext+0avgdata 528maxresident)k
0inputs+0outputs (0major+168minor)pagefaults 0swaps

/tmp # time ./foo /mnt/hdd/xxx 10
posix_fallocate(fd, 0, size_in_GiB << 30): 0 [141 ms]
unlink(filename): 0 [0 ms]
0.00user 3.71system 0:03.83elapsed 96%CPU (0avgtext+0avgdata 528maxresident)k
0inputs+0outputs (0major+168minor)pagefaults 0swaps

/tmp # time ./foo /mnt/hdd/xxx 100
posix_fallocate(fd, 0, size_in_GiB << 30): 0 [1882 ms]
unlink(filename): 0 [0 ms]
0.00user 37.12system 0:38.93elapsed 95%CPU (0avgtext+0avgdata 528maxresident)k
0inputs+0outputs (0major+168minor)pagefaults 0swaps

/tmp # time ./foo /mnt/hdd/xxx 300
posix_fallocate(fd, 0, size_in_GiB << 30): 0 [3883 ms]
unlink(filename): 0 [0 ms]
0.00user 111.38system 1:55.04elapsed 96%CPU (0avgtext+0avgdata 528maxresident)k
0inputs+0outputs (0major+168minor)pagefaults 0swaps


QUESTIONS:

1) Did I provide enough information for someone to reproduce?

2) Is this expected behavior?

3) Are there knobs I can tweak (at FS creation, or at mount time)
to improve the performance of file unlinking?
(Maybe there is a safety/performance trade-off?


My test program:

#define _FILE_OFFSET_BITS 64
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <time.h>

#define BENCH(op) do { \
  struct timespec t0; clock_gettime(CLOCK_MONOTONIC, &t0); \
  int err = op; \
  struct timespec t1; clock_gettime(CLOCK_MONOTONIC, &t1); \
  int ms = (t1.tv_sec-t0.tv_sec)*1000 + (t1.tv_nsec-t0.tv_nsec)/1000000; \
  printf("%s: %d [%d ms]\n", #op, err, ms); } while(0)

int main(int argc, char **argv)
{
  if (argc != 3) { puts("Usage: prog filename size"); return 42; }

  char *filename = argv[1];
  int fd = open(filename, O_CREAT | O_EXCL | O_WRONLY, 0600);
  if (fd < 0) { perror("open"); return 1; }

  long long size_in_GiB = atoi(argv[2]);
  BENCH(posix_fallocate(fd, 0, size_in_GiB << 30));
  BENCH(unlink(filename));
  return 0;
}


-- 
Regards.

Re: After unlinking a large file on ext4, the process stalls for a long time

[John Stoffel]

Mason> I'm using Linux (3.1.10 at the moment) on a embedded system
Mason> similar in spec to a desktop PC from 15 years ago (256 MB RAM,
Mason> 800-MHz CPU, USB).

Sounds like a Raspberry Pi...  And have you investigated using
something like XFS as your filesystem instead?  

Mason> I need to be able to create large files (50-1000 GB) "as fast
Mason> as possible".  These files are created on an external hard disk
Mason> drive, connected over Hi-Speed USB (typical throughput 30
Mason> MB/s).

Really... so you just need to create allocations of space as quickly
as possible, which will then be filled in later with actuall data?  So
basically someone will say "give me 600G of space reservation" and
then will eventually fill it up, otherwise you say "Nope, can't do
it!"

Mason> Sparse files were not an acceptable solution (because the space
Mason> must be reserved, and the operation must fail if the space is
Mason> unavailable).  And filling the file with zeros was too slow
Mason> (typically 35 s/GB).

Mason> Someone mentioned fallocate on an ext4 partition.

Mason> So I create an ext4 partition with
Mason> $ mkfs.ext4 -m 0 -i 1024000 -O ^has_journal,^huge_file /dev/sda1
Mason> (Using e2fsprogs-1.42.10 if it matters)

Mason> And mount with "typical" mount options
Mason> $ mount -t ext4 /dev/sda1 /mnt/hdd -o noexec,noatime
Mason> /dev/sda1 on /mnt/hdd type ext4 (rw,noexec,noatime,barrier=1)

Mason> I wrote a small test program to create a large file, then immediately
Mason> unlink it.

Mason> My problem is that, while file creation is "fast enough" (4 seconds
Mason> for a 300 GB file) and unlink is "immediate", the process hangs
Mason> while it waits (I suppose) for the OS to actually complete the
Mason> operation (almost two minutes for a 300 GB file).

Mason> I also note that the (weak) CPU is pegged, so perhaps this problem
Mason> does not occur on a desktop workstation?

Mason> /tmp # time ./foo /mnt/hdd/xxx 5
Mason> posix_fallocate(fd, 0, size_in_GiB << 30): 0 [68 ms]
Mason> unlink(filename): 0 [0 ms]
Mason> 0.00user 1.86system 0:01.92elapsed 97%CPU (0avgtext+0avgdata 528maxresident)k
Mason> 0inputs+0outputs (0major+168minor)pagefaults 0swaps

Mason> /tmp # time ./foo /mnt/hdd/xxx 10
Mason> posix_fallocate(fd, 0, size_in_GiB << 30): 0 [141 ms]
Mason> unlink(filename): 0 [0 ms]
Mason> 0.00user 3.71system 0:03.83elapsed 96%CPU (0avgtext+0avgdata 528maxresident)k
Mason> 0inputs+0outputs (0major+168minor)pagefaults 0swaps

Mason> /tmp # time ./foo /mnt/hdd/xxx 100
Mason> posix_fallocate(fd, 0, size_in_GiB << 30): 0 [1882 ms]
Mason> unlink(filename): 0 [0 ms]
Mason> 0.00user 37.12system 0:38.93elapsed 95%CPU (0avgtext+0avgdata 528maxresident)k
Mason> 0inputs+0outputs (0major+168minor)pagefaults 0swaps

Mason> /tmp # time ./foo /mnt/hdd/xxx 300
Mason> posix_fallocate(fd, 0, size_in_GiB << 30): 0 [3883 ms]
Mason> unlink(filename): 0 [0 ms]
Mason> 0.00user 111.38system 1:55.04elapsed 96%CPU (0avgtext+0avgdata 528maxresident)k
Mason> 0inputs+0outputs (0major+168minor)pagefaults 0swaps


Mason> QUESTIONS:

Mason> 1) Did I provide enough information for someone to reproduce?

Sure, but you didn't give enough information to explain what you're
trying to accomplish here.  And what the use case is.  Also, since you
know you cannot fill 500Gb in any sorta of reasonable time over USB2,
why are you concerned that the delete takes so long?  

I think that maybe using the filesystem for the reservations is the
wrong approach.  You should use a simple daemon which listens for
requests, and then checks the filesystem space and decides if it can
honor them or not.

Then you just store the files as they get writen...

Mason> 2) Is this expected behavior?

Sure, unlinking a 1Gb file that's been written too means (on EXT4)
that you need to update all the filesystem structures.  Now it should
be quicker honestly, but maybe you're not mounting it with a journal?
And have you tried tuning the filesystem to use larger allocations and
blocks?  You're not going to make alot of files on there obviously,
but just a few large ones.  

Mason> 3) Are there knobs I can tweak (at FS creation, or at mount
Mason> time) to improve the performance of file unlinking?  (Maybe
Mason> there is a safety/performance trade-off?

Sure, there are all kinds of things you can do.  For example, how
many of these files are you expecting to store?  Will you have to be
able to handle writing of more than one file at a time?  Or are they
purely sequential?  

If you are creating a small embedded system to manage a bunch of USB2
hard drives and write data to them with a space reservation process,
then you need to make sure you can actually handle the data throughput
requirements.  And I'm not sure you can.

John

Re: After unlinking a large file on ext4, the process stalls for a long time

[Mason]

(I hope you'll forgive me for reformatting the quote characters
to my taste.)

On 16/07/2014 17:16, John Stoffel wrote:

> Mason wrote:
>
>> I'm using Linux (3.1.10 at the moment) on a embedded system
>> similar in spec to a desktop PC from 15 years ago (256 MB RAM,
>> 800-MHz CPU, USB).
> 
> Sounds like a Raspberry Pi...  And have you investigated using
> something like XFS as your filesystem instead?

The system is a set-top box (DVB-S2 receiver). The system CPU is
MIPS 74K, not ARM (not that it matters, in this case).

No, I have not investigated other file systems (yet).

>> I need to be able to create large files (50-1000 GB) "as fast
>> as possible".  These files are created on an external hard disk
>> drive, connected over Hi-Speed USB (typical throughput 30 MB/s).
> 
> Really... so you just need to create allocations of space as quickly
> as possible,

I may not have been clear. The creation needs to be fast (in UX terms,
so less than 5-10 seconds), but it only occurs a few times during the
lifetime of the system.

> which will then be filled in later with actual data?

Yes. In fact, I use the loopback device to format the file as an
ext4 partition. 

> basically someone will say "give me 600G of space reservation" and
> then will eventually fill it up, otherwise you say "Nope, can't do
> it!"

Right, take a 1000 GB disk,
Reserve(R1 = 300 GB) <- SUCCESS
Reserve(R2 = 300 GB) <- SUCCESS
Reserve(R3 = 300 GB) <- SUCCESS
Reserve(R4 = 300 GB) <- FAIL
Delete (R1)          <- SUCCESS
Reserve(R4 = 300 GB) <- SUCCESS

>> So I create an ext4 partition with
>> $ mkfs.ext4 -m 0 -i 1024000 -O ^has_journal,^huge_file /dev/sda1
>> (Using e2fsprogs-1.42.10 if it matters)
>> 
>> And mount with "typical" mount options
>> $ mount -t ext4 /dev/sda1 /mnt/hdd -o noexec,noatime
>> /dev/sda1 on /mnt/hdd type ext4 (rw,noexec,noatime,barrier=1)
>> 
>> I wrote a small test program to create a large file, then immediately
>> unlink it.
>> 
>> My problem is that, while file creation is "fast enough" (4 seconds
>> for a 300 GB file) and unlink is "immediate", the process hangs
>> while it waits (I suppose) for the OS to actually complete the
>> operation (almost two minutes for a 300 GB file).

[snip performance numbers]

>> QUESTIONS:
>> 
>> 1) Did I provide enough information for someone to reproduce?
> 
> Sure, but you didn't give enough information to explain what you're
> trying to accomplish here.  And what the use case is.  Also, since you
> know you cannot fill 500Gb in any sort of reasonable time over USB2,
> why are you concerned that the delete takes so long?

I don't understand your question. If the user asks to create a 300 GB
file, then immediately realizes than he won't need it, and asks for it
to be deleted, I don't see why the process should hang for 2 minutes.

The use case is
- allocate a large file
- stick a file system on it
- store stuff (typically video files) inside this "private" FS
- when the user decides he doesn't need it anymore, unmount and unlink
(I also have a resize operation in there, but I wanted to get the
basics before taking the hard stuff head on.)

So, in the limit, we don't store anything at all: just create and
immediately delete. This was my test.

> I think that maybe using the filesystem for the reservations is the
> wrong approach.  You should use a simple daemon which listens for
> requests, and then checks the filesystem space and decides if it can
> honor them or not.

I considered using ACTUAL partitions, but there were too many downsides.
NB: there may be several "containers" active at the same time.

>> 2) Is this expected behavior?
> 
> Sure, unlinking a 1Gb file that's been written too means (on EXT4)
> that you need to update all the filesystem structures.

Well creating such a file means updating all the filesystem structures,
yet that operation is 30x faster. Also note that I have not written
ANYTHING to the file; my test did:

  open();
  posix_fallocate();
  unlink();

> Now it should
> be quicker honestly, but maybe you're not mounting it with a journal?

Indeed no, I expected the journal to slow things down.
$ mkfs.ext4 -m 0 -i 1024000 -O ^has_journal,^huge_file /dev/sda1
https://lwn.net/Articles/313514/

Also, the user might format a Flash-based device, and I've read that
journals and Flash-based storage are not a good mix.

> And have you tried tuning the filesystem to use larger allocations and
> blocks?  You're not going to make a lot of files on there obviously,
> but just a few large ones.

Are you suggesting bigalloc?
https://ext4.wiki.kernel.org/index.php/Bigalloc
1. It is not supported by my kernel AFAIU.
2. It is still experimental AFAICT.
3. Resizing bigalloc file systems is not well tested.

>> 3) Are there knobs I can tweak (at FS creation, or at mount
>> time) to improve the performance of file unlinking?  (Maybe
>> there is a safety/performance trade-off?
> 
> Sure, there are all kinds of things you can do.  For example, how
> many of these files are you expecting to store?

I do not support more than 8 containers. (But the drive is used to
store other (mostly large) files.)

This is why I specified "-i 1024000" to mkfs.ext4, to limit the number
of inodes created. Is this incorrect?

What other improvements would you suggest?
(I'd like to get the unlink operation to complete in < 10 seconds.)

> Will you have to be able to handle writing of more than one file
> at a time?  Or are they purely sequential?

All containers may be active concurrently, and since they are proper
file systems, they are written to as the FS drivers sees fit (i.e. not
sequentially). However, the max write throughput is limited to 3 MB/s
(which USB2 should easily manage to handle).

> If you are creating a small embedded system to manage a bunch of USB2
> hard drives and write data to them with a space reservation process,
> then you need to make sure you can actually handle the data throughput
> requirements.  And I'm not sure you can.

AFAIK, the plan is to support only one drive, and not to write faster
than 3 MB/s. I think it should handle it.

Thanks for your insightful questions :-)

Regards.

Re: After unlinking a large file on ext4, the process stalls for a long time

[John Stoffel]

Mason> (I hope you'll forgive me for reformatting the quote characters
Mason> to my taste.)

No problem.

Mason> On 16/07/2014 17:16, John Stoffel wrote:

>> Mason wrote:
>> 
>>> I'm using Linux (3.1.10 at the moment) on a embedded system
>>> similar in spec to a desktop PC from 15 years ago (256 MB RAM,
>>> 800-MHz CPU, USB).
>> 
>> Sounds like a Raspberry Pi...  And have you investigated using
>> something like XFS as your filesystem instead?

Mason> The system is a set-top box (DVB-S2 receiver). The system CPU is
Mason> MIPS 74K, not ARM (not that it matters, in this case).

So it's a slow slow box... and it's only going to handle writing data
at 3Mbs... so why do you insist that the filesystem work at magic
speeds?  

Mason> No, I have not investigated other file systems (yet).

>>> I need to be able to create large files (50-1000 GB) "as fast
>>> as possible".  These files are created on an external hard disk
>>> drive, connected over Hi-Speed USB (typical throughput 30 MB/s).
>> 
>> Really... so you just need to create allocations of space as quickly
>> as possible,

Mason> I may not have been clear. The creation needs to be fast (in UX terms,
Mason> so less than 5-10 seconds), but it only occurs a few times during the
Mason> lifetime of the system.

If this only happens a few times, why do you care how quick the delete
is?  And if it's only happening a few times, why don't you just do the
space reservation OUTSIDE of the filesystem? 

Or do you need to do encryption of these containers and strictly
segrate them?  Basically, implement a daemon which knows how much free
space is on the device, how much is already pre-committed to other
users, and then how much free space there is.  

If the space isn't actually used, then you don't care, because you've
reserved it.  

>> which will then be filled in later with actual data?

Mason> Yes. In fact, I use the loopback device to format the file as an
Mason> ext4 partition. 

Why are you doing it like this?  What advantage does this buy you?  In
any case, you're now slowing things down because you have the overhead
of the base filesystem, which you then create a large file on top of,
which you then mount and format with a SECOND filesystem.  

Instead, you should probably just have a small boot/OS filesystem, and
then put the rest of the storage under LVM control.  At that point,
you can reserve space using 'lvcreate ...' which will succeed or
fail.  If good, create a filesystem in there and use it.  When you
need to delete it, just unmount the LV and just do 'lvdestroy' which
should be much faster, since you won't bother to zero out the blocks.

Now I don't know offhand if lvcreate ontop of a recently deleted LV
volume whill make sure to zero all the blocks, but I suspect so, and
probably only when they're used.

Does this make more sense?  It seems to fit your strange requirements
better...

John


>> basically someone will say "give me 600G of space reservation" and
>> then will eventually fill it up, otherwise you say "Nope, can't do
>> it!"

Mason> Right, take a 1000 GB disk,
Mason> Reserve(R1 = 300 GB) <- SUCCESS
Mason> Reserve(R2 = 300 GB) <- SUCCESS
Mason> Reserve(R3 = 300 GB) <- SUCCESS
Mason> Reserve(R4 = 300 GB) <- FAIL
Mason> Delete (R1)          <- SUCCESS
Mason> Reserve(R4 = 300 GB) <- SUCCESS

>>> So I create an ext4 partition with
>>> $ mkfs.ext4 -m 0 -i 1024000 -O ^has_journal,^huge_file /dev/sda1
>>> (Using e2fsprogs-1.42.10 if it matters)
>>> 
>>> And mount with "typical" mount options
>>> $ mount -t ext4 /dev/sda1 /mnt/hdd -o noexec,noatime
>>> /dev/sda1 on /mnt/hdd type ext4 (rw,noexec,noatime,barrier=1)
>>> 
>>> I wrote a small test program to create a large file, then immediately
>>> unlink it.
>>> 
>>> My problem is that, while file creation is "fast enough" (4 seconds
>>> for a 300 GB file) and unlink is "immediate", the process hangs
>>> while it waits (I suppose) for the OS to actually complete the
>>> operation (almost two minutes for a 300 GB file).

Mason> [snip performance numbers]

>>> QUESTIONS:
>>> 
>>> 1) Did I provide enough information for someone to reproduce?
>> 
>> Sure, but you didn't give enough information to explain what you're
>> trying to accomplish here.  And what the use case is.  Also, since you
>> know you cannot fill 500Gb in any sort of reasonable time over USB2,
>> why are you concerned that the delete takes so long?

Mason> I don't understand your question. If the user asks to create a 300 GB
Mason> file, then immediately realizes than he won't need it, and asks for it
Mason> to be deleted, I don't see why the process should hang for 2 minutes.

Mason> The use case is
Mason> - allocate a large file
Mason> - stick a file system on it
Mason> - store stuff (typically video files) inside this "private" FS
Mason> - when the user decides he doesn't need it anymore, unmount and unlink
Mason> (I also have a resize operation in there, but I wanted to get the
Mason> basics before taking the hard stuff head on.)

Mason> So, in the limit, we don't store anything at all: just create and
Mason> immediately delete. This was my test.

>> I think that maybe using the filesystem for the reservations is the
>> wrong approach.  You should use a simple daemon which listens for
>> requests, and then checks the filesystem space and decides if it can
>> honor them or not.

Mason> I considered using ACTUAL partitions, but there were too many downsides.
Mason> NB: there may be several "containers" active at the same time.

>>> 2) Is this expected behavior?
>> 
>> Sure, unlinking a 1Gb file that's been written too means (on EXT4)
>> that you need to update all the filesystem structures.

Mason> Well creating such a file means updating all the filesystem structures,
Mason> yet that operation is 30x faster. Also note that I have not written
Mason> ANYTHING to the file; my test did:

Mason>   open();
Mason>   posix_fallocate();
Mason>   unlink();

>> Now it should
>> be quicker honestly, but maybe you're not mounting it with a journal?

Mason> Indeed no, I expected the journal to slow things down.
Mason> $ mkfs.ext4 -m 0 -i 1024000 -O ^has_journal,^huge_file /dev/sda1
Mason> https://lwn.net/Articles/313514/

Mason> Also, the user might format a Flash-based device, and I've read that
Mason> journals and Flash-based storage are not a good mix.

>> And have you tried tuning the filesystem to use larger allocations and
>> blocks?  You're not going to make a lot of files on there obviously,
>> but just a few large ones.

Mason> Are you suggesting bigalloc?
Mason> https://ext4.wiki.kernel.org/index.php/Bigalloc
Mason> 1. It is not supported by my kernel AFAIU.
Mason> 2. It is still experimental AFAICT.
Mason> 3. Resizing bigalloc file systems is not well tested.

>>> 3) Are there knobs I can tweak (at FS creation, or at mount
>>> time) to improve the performance of file unlinking?  (Maybe
>>> there is a safety/performance trade-off?
>> 
>> Sure, there are all kinds of things you can do.  For example, how
>> many of these files are you expecting to store?

Mason> I do not support more than 8 containers. (But the drive is used to
Mason> store other (mostly large) files.)

Mason> This is why I specified "-i 1024000" to mkfs.ext4, to limit the number
Mason> of inodes created. Is this incorrect?

Mason> What other improvements would you suggest?
Mason> (I'd like to get the unlink operation to complete in < 10 seconds.)

>> Will you have to be able to handle writing of more than one file
>> at a time?  Or are they purely sequential?

Mason> All containers may be active concurrently, and since they are proper
Mason> file systems, they are written to as the FS drivers sees fit (i.e. not
Mason> sequentially). However, the max write throughput is limited to 3 MB/s
Mason> (which USB2 should easily manage to handle).

>> If you are creating a small embedded system to manage a bunch of USB2
>> hard drives and write data to them with a space reservation process,
>> then you need to make sure you can actually handle the data throughput
>> requirements.  And I'm not sure you can.

Mason> AFAIK, the plan is to support only one drive, and not to write faster
Mason> than 3 MB/s. I think it should handle it.

Mason> Thanks for your insightful questions :-)

Mason> Regards.

Re: After unlinking a large file on ext4, the process stalls for a long time

[Mason]

On 16/07/2014 22:18, John Stoffel wrote:

> Mason wrote:
>
>> The system is a set-top box (DVB-S2 receiver). The system CPU is
>> MIPS 74K, not ARM (not that it matters, in this case).
> 
> So it's a slow slow box... and it's only going to handle writing data
> at 3 MBps...

What do you mean "it's only going to handle writing data at 3 MBps"?
It's a good thing that I am nowhere near the peak throughput! I have
tested the system for hours, and it can handle 30 MB/s with only a
few minor hiccups (unexplained, so far).

> so why do you insist that the filesystem work at magic speeds?

Why do you say that deleting an empty 300-GB fallocated file in
less than 10 seconds is "working at magic speeds"? What numbers
do you get for this benchmark? I would expect to get roughly
similar numbers for creation and deletion. Why aren't you
surprised by the deletion numbers? You know something I don't.
So please share with me.

>> I may not have been clear. The creation needs to be fast (in UX terms,
>> so less than 5-10 seconds), but it only occurs a few times during the
>> lifetime of the system.
> 
> If this only happens a few times, why do you care how quick the delete
> is?  And if it's only happening a few times, why don't you just do the
> space reservation OUTSIDE of the filesystem?

I care because the delete operation is done when the user asks
for it, and it hangs the UI for 2 minutes. Isn't that reason
enough to care?

I don't know what you mean by reservation OUTSIDE of the FS.
The user supplies the external HDD, and the sizes to reserve
are known at run-time (sent in the broadcast signal).

> Or do you need to do encryption of these containers and strictly
> segrate them?  Basically, implement a daemon which knows how much free
> space is on the device, how much is already pre-committed to other
> users, and then how much free space there is.

I don't think you've thought this through...
You propose to have a daemon that will mediate every file system
write to the external HDD. That means that the application has
to explicitly be coded to talk to the daemon. (My solution is
transparent to the app.) Or did you have some kind of interposition
of write system calls? Anyway, this code would be duplicating the
bean counting done inside the file system driver. (I considered
using quotas, but I didn't see how to make it work as required.)

Note that the OS and main app reside in Flash (ubifs). And there
are also a couple tmpfs. There are about 15 mount points (most
of them pseudo file systems, though).

> Why are you doing it like this?  What advantage does this buy you?  In
> any case, you're now slowing things down because you have the overhead
> of the base filesystem, which you then create a large file on top of,
> which you then mount and format with a SECOND filesystem.

Write performance is secondary. I was just providing insight
into what I planned to do with the large files, but the
performance problem of unlinking occurs *even when nothing
was done to the file*. (Please don't get distracted by the
FS-within-FS gizmo.)

> Instead, you should probably just have a small boot/OS filesystem, and
> then put the rest of the storage under LVM control.  At that point,
> you can reserve space using 'lvcreate ...' which will succeed or
> fail.  If good, create a filesystem in there and use it.  When you
> need to delete it, just unmount the LV and just do 'lvdestroy' which
> should be much faster, since you won't bother to zero out the blocks.

I didn't look closely at the LVM solution, because space in
Flash is tight, and I had the prejudice that LVM was large,
as a server/workstation solution.

[huge snip of my other questions]

I will try to test XFS on the STB, and also run my ext4 test
on my workstation, see if get the same disappointing results.

Regards.