5

I have a workload with extremely high write burst rates for short periods of times. The target disks are rather slow, but I have plenty of RAM and very tolerant to instantaneous data loss.

I've tried tuning vm.dirty_ratio to maximize the use of free RAM space to be used for dirty pages. 

# free -g
              total        used        free      shared  buff/cache   available
Mem:            251           7         213           3          30         239
Swap:             0           0           0

# sysctl -a | grep -i dirty
vm.dirty_background_bytes = 0
vm.dirty_background_ratio = 5
vm.dirty_bytes = 0
vm.dirty_expire_centisecs = 90000
vm.dirty_ratio = 90

However, it seems I'm still encountering some writeback throttling based on the underlying disk speed. How can I disable this? 

# dd if=/dev/zero of=/home/me/foo.txt bs=4K count=100000 oflag=nonblock
100000+0 records in
100000+0 records out
409600000 bytes (410 MB) copied, 10.2175 s, 40.1 MB/s

As long as there is free memory and the dirty ratio has not yet been exceeded - I'd like to write at full speed to the page cache.

Improve this question
1

1 Answer 1

Reset to default
1

The problem is not the dirty-throttling, but the immediate syncing. You can check it with the bo column in vmstat 1 - I get:

   -----io----
   bi    bo
    0     0
    0     0
    0  4000
    0     0
    ...

after dd if=/dev/zero of=16/test bs=4k count=1000. (The vmstat output is just 1K-"units" - /proc/diskstats has 512B-units)

Adding conv=notrunc avoids that, as does a rm before. See sourcejedi's comment.

The ext4 mount option noauto_da_alloc exactly is meant for that: no extra sync for these "write-file-anew" operations. man ext4 has some good examples. The question goes back to what should "write" or "save" file mean in an editor, and how to make that (power-) fail-safe.

You can also check the other side: the dirty pages growing:

]# grep nr_dirty /proc/vmstat      
nr_dirty 7886
nr_dirty_threshold 427666
nr_dirty_background_threshold 170815

The thresholds are the ratios applied to the current available memory. Only since I quite disabled the periodic writebacks I see more than a dozen dirty pages.

vim

It is really a complex situation; vim has at least two crucial options: fs and wb: filesync after write and overwrite via backup.

On ext2 it is enough to set nofs in vim. On ext4 (with default delayed allocation) the noauto_da_alloc mount option is also needed to prevent an immediate syncing, this time by the filesystem.

(with dd you have the option to (over)write only a part of a file, unlike cp or vim)

Improve this answer

You must log in to answer this question.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.