How to transparently cache any directory or mounted file system for reads and write back

cachecloudfuse

Say I mount some cloud storage (Amazon Cloud Drive in my case) with a FUSE client at /mnt/cloud. But because reading and writing files directly to /mnt/cloud is slow because it has to go over the internet, I want to cache the files that I'm reading from and writing to cloud storage. Since I might be writing a lot of data at a time, the cache should sit on my disk and not in RAM. But I don't want to replicate the entire cloud storage on my disk, because my disk may be too small.

So I want to have a cached view into /mnt/cloud mounted at /mnt/cloud_cache, which uses another path, say /var/cache/cloud as the caching location.

If I now read /mnt/cloud_cache/file, I want the following to happen:

Check whether file is cached at /var/cache/cloud/file.

If cached: Check file in cache is up-to-date by fetching modtime and/or checksum from /mnt/cloud. If it's up-to-date, serve the file from the cache, otherwise go to 2.
If not cached or cache is out-of-date: Copy /mnt/cloud/file to /var/cache/cloud/file and serve it from the cache.

When I write to /mnt/cloud_cache/file, I want this to happen:

Write to /var/cache/cloud/file and record in a journal that file needs to be written back to /mnt/cloud
Wait for writing to /var/cache/cloud/file to be done and/or previous write backs to /mnt/cloud to be completed
Copy /var/cache/cloud/file to /mnt/cloud

I have the following requirements and constraints:

Free and open source
Ability to set cache an arbitrary cache location
Ability to cache an arbitrary location (probably some FUSE mount point)
Transparent caching, i.e. using /mnt/cloud_cache is transparent to the caching mechanism and works like any other mounted file system
Keeping a record of what needs to be written back (the cache might get a lot of data that needs to be written back to the original storage location over the course of days)
Automatic deletion of cached files that have been written back or have not been accessed in a while
Consistency (i.e. reflecting external changes to /mnt/cloud) isn't terribly important, as I will probably have only one client accessing /mnt/cloud at a time, but it would be nice to have.

I've spent quite some time looking for existing solutions, but haven't found anything satisfactory.

FS-Cache and CacheFS (https://www.kernel.org/doc/Documentation/filesystems/caching/fscache.txt) seems to only work with nfs or afs file systems and I don't know how to make it cache another FUSE file system or any general directory.
bcache (https://bcache.evilpiepirate.org/) seems to only work with block devices, i.e. couldn't cache another FUSE file system
gcsfuse (https://github.com/GoogleCloudPlatform/gcsfuse) I think this does exactly what I want, but it's integrated with Google Cloud Storage. To make it work in general, I would have to hack it and change any accesses to GCS to local file accesses in the given mount-point or accesses to Amazon Cloud Drive

Best Answer

Try using catfs, a generic fuse caching filesystem I'm currently working on.

Potential Method #1 - F_DROP_CACHES

I found a method from 2012 that discusses a proposed patch to the Linux kernel in this mail thread titled: Re: [RFC Patch] fs: implement per-file drop caches.

excerpt

Cong> This is a draft patch of implementing per-file drop caches.

Interesting. So can I do this from outside a process? I'm a SysAdmin, so my POV is from noticing, finding and fixing performance problems when the system is under pressure.
Cong> It introduces a new fcntl command  F_DROP_CACHES to drop  
Cong> file caches of a specific file. The reason is that currently  
Cong> we only have a system-wide drop caches interface, it could  
Cong> cause system-wide performance down if we drop all page caches  
Cong> when we actually want to drop the caches of some huge file.
How can I tell how much cache is used by a file? And what is the performance impact of this when run on a busy system? And what does this patch buy us since I figure the VM should already be dropping caches once the system comes under mem pressure...

Cong> Below is small test case for this patch:

The thread includes both a testcase and the actual patch to several files within the Linux kernel which adds an additional function to fs/drop_caches.c called drop_pagecache_file(struct file *filp). This function is then accessible through the frontend tool, fnctl.c via the command F_DROP_CACHES. This case calls this function:

file_drop_caches(filp, arg);

Which handles the dropping of all the caches associated with the given file. From the file include/linux/mm.h:

void file_drop_caches(struct file *filp, unsigned long which);

So this can be used?

I found no evidence that this patch ever made its way into the main Linux kernel code repository, so this option would appear to be available, only if you're willing to recompile the Linux kernel yourself.

Potential Method #2 - Using dd

In that same thread, another user mentions a completely different methodology that makes use of dd.

The following is excerpt from that email

This is useful functionality. Though isn't it already provided with POSIX_FADV_DONTNEED? This functionality was added to GNU dd (8.11) a year ago.
Here are the examples from that patch:
Advise to drop cache for whole file
 $ dd if=ifile iflag=nocache count=0
Ensure drop cache for the whole file
 $ dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0
Drop cache for part of file
 $ dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null
Stream data using just the read-ahead cache
 $ dd if=ifile of=ofile iflag=nocache oflag=nocache

Testing it out

I wasn't 100% positive how to test this out but I came up with the following approach.

make a 100MB file

$ dd if=/dev/urandom of=sample.txt bs=100M count=1

trace file accesses using fatrace
```
$ sudo fatrace | grep sample.txt
```
run top so we can monitor memory usage, note amount free.
```
$ top
```
open file, note amount of free memory now. Note the fatrace of the file sample.txt.
```
$ cat sample.txt > /dev/null
```

drop the file from memory, note amount of free memory now. Note the output of fatrace.

$ sudo dd of=/home/saml/tst/162600/sample.txt \
    oflag=nocache conv=notrunc,fdatasync count=0

Example

In terminal #1:

$ dd if=/dev/urandom of=sample.txt bs=100M count=1
1+0 records in
1+0 records out
104857600 bytes (105 MB) copied, 7.37996 s, 14.2 MB/s

$ ls -l sample.txt 
-rw-rw-r--. 1 saml saml 104857600 Oct 17 22:54 sample.txt

In terminal #2:

$ top
...
KiB Mem:   7968336 total,  6900956 used,  1067380 free,   267080 buffers
...

In terminal #3:

$ sudo fatrace | grep sample.txt

Now open the file, sample.txt, and note the amount of RAM. In terminal #1.

$ cat sample.txt > /dev/null

In terminal #2:

KiB Mem:   7968336 total,  7011896 used,   956440 free,   267336 buffers

Notice the output of fatrace in terminal #3:

cat(25940): R /home/saml/tst/162600/sample.txt
cat(25940): R /home/saml/tst/162600/sample.txt
cat(25940): RC /home/saml/tst/162600/sample.txt

Now remove the file from RAM, in terminal #4:

$ sudo dd of=/home/saml/tst/162600/sample.txt \
    oflag=nocache conv=notrunc,fdatasync count=0

Note the output of fatrace in terminal #2:

dd(26229): O /home/saml/tst/162600/sample.txt
dd(26229): CW /home/saml/tst/162600/sample.txt

Note the RAM in terminal #3:

KiB Mem:   7968336 total,  6908364 used,  1059972 free,   267364 buffers

So it would seem that all of the that was consumed by the file in RAM is freed.

Potential Method #3 - python-fadvise

Thanks to a comment by @frostchutz, there's another tool, a Python script, named [pyadvise][4] which provides a much simpler interface than the above dd methods. This script makes use of the same posix_fadvise(2) interface.

Example

$ sudo pyadvise --help
Usage: 
    pyadvise [options] [FILE]..

Options:
  -h, --help        show this help message and exit
  -w, --willneed    The specified files will be accessed in the near future
  -s, --sequential  The application expects to access the specified files
                    sequentially (with lower offsets read before higher ones)
  -d, --dontneed    The specified files will not be accessed in the near
                    future
  -r, --random      The specified files will be accessed in random order
  -o, --noreuse     The specified files will be accessed only once. Under
                    Linux, this operation is a no-op; see contrib/copyfileobj-
                    fadvise.py in the python-fadvise source tree for an
                    example on how to achieve approximately the same effect
  -n, --normal      Indicates that the application has no advice to give about
                    its access pattern for the specified files. If no advice
                    is given for an open file, this is the default assumption
  -v, --verbose     Explain what is being done

And if we repeat the above test and use pyadvise in place of dd:

$ pyadvise -d /home/saml/tst/162600/sample.txt

I noticed an identical drop in the RAM being consumed as before when I used dd.

Ubuntu – How to tune Ubuntu for running from a flash drive

USB thumbdrive raw performance

For starters I'd take a look at the USB thumb drive just to make sure that it's operating at an acceptable level. You can use the following 2 commands to measure it's performance:

$ hdparm -t /dev/sdb

$ dd count=100 bs=1M if=/dev/zero of=/media/disk/test oflag=sync

Substitute in the appropriate HDD info for your given setup.

File system

Also take a look at this article on increasing performance of USB thumbdrives from the Ubuntu help site.

Best Answer

Related Solutions

Linux – Drop a specific file from the Linux filesystem cache