Linux – Clearing Caches

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by Lisa

I encountered some documentation at work that provided a process for clearing caches. It wasn’t wrong per se, but it showed a lack of understanding of what was being performed. I enhanced our documentation to explain what was happening and why the series of commands was redundant. Figured I’d post my revisions here in case they’re useful for someone else.

Only clean caches can be dropped — dirty ones need to be written somewhere before they can be dropped. Before dropping caches, flush the file system buffer using sync — this tells the kernel to write dirty cache pages to disk (or, well, write as many as it can). This will maximize the number of cache pages that can be dropped. You don’t have to run sync, but doing so optimizes your subsequent commands effectiveness.

Page cache is memory that’s held after reading a file. Linux tends to keep the files in cache on the assumption that a file that’s been read once will probably be read again. Clear the pagecache using echo 1 > /proc/sys/vm/drop_caches — this is the safest to use in production and generally a good first try.

If clearing the pagecache has not freed sufficient memory, proceed to this step. The dentries (directory cache) and inodes cache are memory held after reading file attributes (run strace and look at all of those stat() calls!). Clear the dentries and inodes using echo 2 > /proc/sys/vm/drop_caches — this is kind of a last-ditch effort for a production environment. Better than having it all fall over, but things will be a little slow as all of the in-flight processes repopulate the cached data.

You can clear the pagecache, dentries, and inodes using echo 3 > /proc/sys/vm/drop_caches — this is a good shortcut in a non-production environment. But, if you’re already run 1 and 2 … well, 3 = 1+2, so clearing 1, 2, and then 3 is repetitive.

 

Another note from other documentation I’ve encountered — you can use sysctl to clear the caches, but this can cause a deadlock under heavy load … as such, I don’t do this. The syntax is sysctl -w vm.drop_caches=1 where the number corresponds to the 1, 2, and 3 described above.

 

Using Excel To Decrypt ROT

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by Lisa

Anya has made us a scavenger hunt complete with clues — clues that are encrypted! While working out a ROT-1 or ROT-2 decryption isn’t terribly difficult, it was getting a little time consuming [especially when what was advertised as ROT-1 was actually ROT-(25) and I had to do it all over again!], so I made a quick spreadsheet to decrypt strings. The letter pairs are in the first two columns, then VLOOKUP is used to convert the printed character to the actual one. I found a trick card!

ElasticSearch – Useful API Commands

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by Lisa

In all of these examples, the copy/paste text uses localhost and port 9200. Since some of my sandboxes don’t use the default port, some of the example outputs will use a different port. Obviously, use your hostname and port. And, if your ES instance requires authentication, add the “-u” option with the user (or user:password … but that’s not a good idea outside of sandboxes as the password is then stored to the shell history). If you are using https for the API endpoint, you may also need to add the “-k” option to establish an untrusted SSL connection (e.g. the CA isn’t trusted by your OS).

curl -k -u elastic https://localhost...

Listing All Indices

Use the following command to list all of the indices in the ES system:

curl http://localhost:9200/_cat/indices?v

Listing All Templates

Use the following command to list all of the templates:
curl http://localhost:9200/_cat/templates?pretty

Explain Shard Allocation

I was asked to help get a ELK installation back into working order — one of the things I noticed is that all of the indices were yellow. The log file showed allocation errors. This command reported on the allocation decision that was being made. In the case I was looking at, the problem became immediately obvious — it was a single node system and 1 replica was defined. The explanation was that the shard could not be stored because it already existed in that place.

curl http://localhost:9200/_cluster/allocation/explain

If the maximum number of allocation retries has been exceeded, you can force ES to re-try allocation (as an example, a disk was full for an extended period of time but space has been cleared and everything should work now)

curl http://localhost:9200/_cluster/reroute?retry_failed=true
Set the Number of Replicas for a Single Index

Once I identified that the single node ELK instance had indices configured

curl -X PUT \
http://127.0.0.1:9200/logstash-2021.05.08/_settings \
-H 'cache-control: no-cache' \
-H 'content-type: application/json' \
-d '{"index" : {"number_of_replicas" : 0}}
'

Add an Alias to an Index

To add an alias to an existing index, use PUT /<indexname>/_alias/<aliasname> — e.g.

PUT /ljr-2022.07.05/_alias/ljr

2022 Maple Season

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by Lisa

Well, the 2022 maple season is over — I think our taps have dried up because we’ve had a few freeze/thaw days and haven’t really yielded an appreciable amount of sap. We only got like 2.5 gallons of syrup this year — much less than expected … and we need to be ready to tap in January next year when the first week of freeze/thaw hits. While I love the flavor of late-season syrup, we’re getting way too many warm days in March for good sap production.

Docker – Changing an Existing Container

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by Lisa

I’ll start by acknowledging that, of course, you could just redeploy the container with the settings you want now. The whole point of containerized development is that anything “good” should either be part of the deployment settings or data persisted outside of the container. So, in theory, redeploying the container every day shouldn’t really be detectable. Even when you didn’t deploy the original container (i.e. you don’t have the Dockerfile or docker run command handy to tweak as needed), you can reverse engineer what you need from docker inspect. But sometimes? It’s quicker/easier/more convenient to just fix what you need to within the existing container. And it is possible to do so.

The trickiest part is finding the right file to edit.

# cd into docker container definition folder
cd /var/lib/docker/containers/
# find the guid for the container you want to edit
docker ps
# Find the corresponding folder name
ls -al | grep bc9dc66882af
# cd into that folder
cd bc9dc66882af18f59c209faf10031fe21765571d0a2fe4a32a768a1d52cc1e53
# Edit the config.v2.json file for the container
vi config.v2.json
# And, finally, restart docker
systemctl stop docker
systemctl start docker

Tiny Turkey Army, Take Two

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by Lisa

The new turkeys arrived today — last year, USPS shipping was a horrible experience. This year, I called a few hatcheries to confirm they’ve been able to delivery healthy, happy poults. Meyers said they hadn’t had delivery problems, so we ordered 20 Black Spanish turkeys from them. They shipped yesterday, the shipping notice was delivered overnight, and the USPS clerk called at 6:30 this morning to let me know they arrived. Wow, was that early!

We got all the little ones into their brooder, fed, and watered (having more healthy birds seems to help because one little guy eats or drinks and a whole flock of little ones come over and copy it).