How to purge disk I/O caches on Linux?
Sounds like you want the sync command, or the sync() function.
If you want disk cache flushing: echo 3 | sudo tee /proc/sys/vm/drop_caches
sync is 100% unrelated. I’m talking about long-lived multi-GB read caches, not trivial amounts of short-lived unwritten data which sync deals with (and which gets written to disk every 10 or so seconds anyway).
Actually even though you tell the OS to drop the caches, the hard drive doesn’t have to 🙂 The only way to force this to happen is to power down the machine, found this out the hard way (on disk cache)
@ChrisDennett I do the same thing when my code doesn’t compile, just to make sure the compiler knows what I’m doing, maybe it will get the message too
Please consider switching the 2 sentences of your answer. The drop_caaches part is what I wanted when I came to this page.
# sync # (move data, modified through FS -> HDD cache) + flush HDD cache # echo 3 > /proc/sys/vm/drop_caches # (slab + pagecache) -> HDD (https://www.kernel.org/doc/Documentation/sysctl/vm.txt) # blockdev --flushbufs /dev/sda # hdparm -F /dev/sda # NEXT COMMAND IS NOT FOR BENCHMARKING: # should be run before unplug, flushes everything possible guaranteed. # echo 1 > /sys/block/sdX/device/delete You may use strace to see that these are three different syscalls
Also, it may be desirable to turn off HDD cache using hdparm, not sure what thing you benchmarking.
In any way, you cannot prevent HDD to cache last 64/32/16 MB of recently used data. In order to kill that cache, just write some amount of zeroes (and flush) + read some unrelated place from HDD. This is required since cache may be divided to read-part and write-part. After that you can benchmark HDD.
blockdev —flushbufs /dev/sda works with my USB drive, but has no effect with SATA SSD drive. echo 3 | sudo tee /proc/sys/vm/drop_caches works with both drives.
The last command removes the disk from the kernel. You definitely won’t be able to benchmark the disk after (or use the disk in any way) and if the disk is in use it’s dangerous. There’s no reason to include it along with the other commands. Here’s an example of when to use this command and what it does: access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/…
Disk cache purging: echo 3 | sudo tee /proc/sys/vm/drop_caches
Writing to this will cause the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free.
To free pagecache:
echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:
echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:
echo 3 > /proc/sys/vm/drop_caches
As this is a non-destructive operation, and dirty objects are not freeable, the user should run «sync» first in order to make sure all cached objects are freed.
Short good enough answer: (copy paste friendly)
DISK=/dev/sdX # /proc/sys/vm/drop_caches blockdev --flushbufs $DISK hdparm -F $DISK Explanation:
sync : From the man page: flush file system buffers. Force changed blocks to disk, update the super block.
echo 3 > /proc/sys/vm/drop_cache : from the kernel docs this will cause the kernel to drop clean caches
blockdev —flushbufs /dev/sda : from the man page: call block device ioctls [to] flush buffers.
hdparm -F /dev/sda : from the man page: Flush the on-drive write cache buffer (older drives may not implement this)
Although the blockdev and hdparm commands look similar according to an answer above they issue different ioctls to the device.
Long probably better way:
(I’ll assume that you have formatted the disk but you can adapt these commands if you want to write directly to the disk)
Run this only once before the 1st benchmark:
Run this every time you want to empty the caches:
DISK=/dev/sdX # /proc/sys/vm/drop_caches blockdev --flushbufs $DISK hdparm -F $DISK # read the file with pseudo-random data to fill any read-cache # the disk may have with garbage dd if=$MOUNT/temp-hddread.tmp of=/dev/null Explanation:
The disk will probably have some H/W cache. Some disks by design or due to bugs may not clear their caches when you issue the blockdev and hdparm commands. To compensate we write and read pseudo-random data hopping to fill these caches so that any cached data are removed from them. How much data you need to fill the cache depends on its size. In the commands above I’m using dd to read/write 16*64MB=1024MB, adjust the arguments if your HDD may have bigger cache (data sheets and experimentation are your friend and it doesn’t hurt to specify values above the actual size of the cache). I’m using /dev/urandom as a source for random data because it’s fast and we don’t care about true randomness (we only care for high entropy because the disk firmware may be using compression before storing data to the cache). I’m creating /mnt/test/temp-hddread.tmp from the start and use it every time I want to read enough random data. I’m creating and deleting /mnt/test/temp-hddwrite.tmp each time I want to write enough random data.
I’ve wrote this answer based on the best parts of the existing answers.
How to Clear RAM Memory Cache, Buffer and Swap Space on Linux
Like any other operating system, GNU/Linux has implemented memory management efficiently and even more than that. But if any process is eating away your memory and you want to clear it, Linux provides a way to flush or clear ram cache.
How to Clear Cache in Linux?
Every Linux System has three options to clear cache without interrupting any processes or services.
1. Clear PageCache only.
# sync; echo 1 > /proc/sys/vm/drop_caches
2. Clear dentries and inodes.
# sync; echo 2 > /proc/sys/vm/drop_caches
3. Clear pagecache, dentries, and inodes.
# sync; echo 3 > /proc/sys/vm/drop_caches
Explanation of the above command.
sync will flush the file system buffer. Command Separated by “;” run sequentially. The shell waits for each command to terminate before executing the next command in the sequence. As mentioned in the kernel documentation, writing to drop_cache will clean cache without killing any application/service, command echo is doing the job of writing to file.
If you have to clear the disk cache, the first command is safest in enterprise and production as “. echo 1 > ….” will clear the PageCache only. It is not recommended to use the third option above “. echo 3 >” in production until you know what you are doing, as it will clear pagecache, dentries, and inodes.
Is it a good idea to free Buffer and Cache in Linux that might be used by Linux Kernel?
Free Buffer and Cache in Linux
When you are applying various settings and want to check, if it is actually implemented specially on the I/O-extensive benchmark, then you may need to clear the buffer cache. You can drop cache as explained above without rebooting the System i.e., no downtime required.
Linux is designed in such a way that it looks into the disk cache before looking onto the disk. If it finds the resource in the cache, then the request doesn’t reach the disk. If we clean the cache, the disk cache will be less useful as the OS will look for the resource on the disk.
Moreover, it will also slow the system for a few seconds while the cache is cleaned and every resource required by OS is loaded again in the disk cache.
Now we will be creating a shell script to auto clear RAM cache daily at 2 am via a cron scheduler task. Create a shell script clearcache.sh and add the following lines.
#!/bin/bash # Note, we are using "echo 3", but it is not recommended in production instead use "echo 1" echo "echo 3 > /proc/sys/vm/drop_caches"
Set execute permission on the clearcache.sh file.
Now you may call the script whenever you are required to clear the ram cache.
Now set a cron to clear RAM cache every day at 2 am. Open crontab for editing.
Append the below line, save and exit to run it at 2 am daily.
0 2 * * * /path/to/clearcache.sh
For more details on how to cron a job, you may like to check our article on 11 Cron Scheduling Jobs.
Is it a good idea to auto clear the RAM cache on the production server?
Clear RAM Cache on Linux Production Server?
No! it is not. Think of a situation when you have scheduled the script to clear ram cache every day at 2 am. Every day at 2 am the script is executed and it flushes your RAM cache. One day for whatsoever reason may be more than expected users are online on your website and seeking resources from your server.
At the same time, the scheduled script runs and clears everything in the cache. Now all the users are fetching data from the disk. It will result in a server crash and corrupt the database. So clear ram-cache only when required, and known your footsteps, else you are a Cargo Cult System Administrator.
How to Clear Swap Space in Linux?
If you want to clear Swap space, you may like to run the below command.
Also, you may add the above command to a cron script above, after understanding all the associated risks.
Now we will be combining both above commands into one single command to make a proper script to clear RAM Cache and Swap Space.
# echo 3 > /proc/sys/vm/drop_caches && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared' OR $ su -c "echo 3 >'/proc/sys/vm/drop_caches' && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared'" root
After testing both the above commands, we will run the command “free -h” before and after running the script and will check the cache.
That’s all for now, if you liked the article, don’t forget to provide us with your valuable feedback in the comments to let us know, what you think is a good idea to clear ram cache and buffer in production and Enterprise?
How to clean /var/cache?
The space is used by /var/cache/polipo (2.7G). How can I clean this up safely?
I tried restarts=>didn’t work
Used bleachbit=>the space is not detected in the cleanup preview PS: I did rm -rf youtube inside /var/cache/polipo and it freed up 2G space. Dunno if it was safe though
6 Answers 6
sudo apt-get autoclean sudo apt-get autoremove All these were already done. The problem is in /var/cache. I want to know how to clean /var/cache safely.
Polipo, a web caching program may store a lot of data in an on-disk cache.
One way to clear this up is to issue the command sudo polipo -x — this will cause polipo to clear the local disk cache.
This is the ONLY right answer to this question (regarding polipo)! Why the hell does it not already have enough votes to make that clear.
The most powerful of all commands to clean the cache for command line users is of course
Which will also delete all the cached files.
I don’t know what this command did but it didn’t clear /var/cache, space is nearly double after I ran it!
Try cleaning ubuntu unnecesarry files using bleachbit. It is a tool that will help you clean your cache, temp files, cookies and it has other features also.
sudo apt-get install bleachbit It sounds so obvious, and yet chances are you haven’t done this.
By default Ubuntu keeps every update it downloads and installs in a cache on your disk, just in case you ever need it again.
This is useful if you regularly add and remove apps, find yourself needing to reconfigure/reinstall a specific package, or simply have a poor connection.
But the flip side is that the apt package cache can quickly swell to several hundred MBs. This command tells you how big your apt cache is:
du -sh /var/cache/apt/archives To clean the apt cache on Ubuntu simply run the following command.
The apt clean command removes ALL packages kept in the apt cache, regardless of age or need. If you’re on a slow, capped or intermittent connection you may want to consider skipping this step.