Is it a good idea to use Linux swap partition/file with rock storage?

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Is it a good idea to use Linux swap partition/file with rock storage?

duanyao
Hi,

I notice that squid's rock storage uses large (and fixed) amount of
shared memory even if it is not accessed. It's estimated as
110byte/slot, so for a 256GB rock storage with 16KB slot, the memory
requirement is about 1.7GB, which is quite large.

So my questions are:

1. Is there a way to reduce memory usage of rock storage?

2. On Linux squid puts its shared memory in /dev/shm, which can be
backed by swap partition/file. Is it a good idea to use swap
partition/file with rock storage to save some physical memory?

3. For rock storage, are /dev/shm/squid* frequently and randomly
written? If the Linux swap is on SSD, will this causes
performance/lifetime issues?

Regards,

Duan Yao

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Re: Is it a good idea to use Linux swap partition/file with rock storage?

Amos Jeffries
Administrator
On 19/09/17 18:25, duanyao wrote:

> Hi,
>
> I notice that squid's rock storage uses large (and fixed) amount of
> shared memory even if it is not accessed. It's estimated as
> 110byte/slot, so for a 256GB rock storage with 16KB slot, the memory
> requirement is about 1.7GB, which is quite large.
>
> So my questions are:
>
> 1. Is there a way to reduce memory usage of rock storage?
>

Reducing the cache size is the only thing that will do that.

For the entire time your Squid is running it is adding to the cache
contents. The rate of growth decreases over time, but will only ever
stop growing if the cache reaches 100% full.

So going out of your way to make it use less memory during that warm-up
phaze is pointless long-term. The memory *is* needed and not having it
available for use with zero advance notice will lead to serious
performance problems, up to and including DoS vulnerability in your proxy.

For general memory reduction see the FAQ:
<https://wiki.squid-cache.org/SquidFaq/SquidMemory#What_can_I_do_to_reduce_Squid.27s_memory_usage.3F>


> 2. On Linux squid puts its shared memory in /dev/shm, which can be
> backed by swap partition/file. Is it a good idea to use swap
> partition/file with rock storage to save some physical memory?
>

Definitely No. The cache index has an extremely high rate of churn and a
large number of random location reads per transaction. If any of it ever
gets pushed out to a swap disk/file the proxy operational speed
undergoes a performance reduction of 3-4 orders of magnitude. eg. 50GBps
-> 2MBps.


> 3. For rock storage, are /dev/shm/squid* frequently and randomly
> written? If the Linux swap is on SSD, will this causes
> performance/lifetime issues?
>

see the answer to (2).

Squid stresses disks in ways vastly different to what manufacturers
optimize the hardware to handle. The HTTP caches have a very high
write-to-read ratio. No disk actually survives more than a fraction of
its manufacturer advertised lifetime. This problem is less visible with
HDD due to their naturally long lifetimes.

Specific to your question, due to the churn mentioned in (2) using a
disk as storage location for the cache index faces it with the worst of
both worlds - very high read throughput and even higher write
throughput. SSD avoid (some of) the speed problem, but at cost of
shorter lifetimes. So the churn is much more relevant and perhapse
costly in hardware replacements.

YMMV depending on the specific SSD model and how it is designed to cope
with dead sectors - but it is guaranteed to wear out much faster than
advertised.

Amos
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