# What determines the size of SSD cache to install?

**URL:** <https://discuss.elastic.co/t/what-determines-the-size-of-ssd-cache-to-install/203129>\
**Category:** Elasticsearch\
**Created:** [October 11, 2019, 1:12am UTC](https://discuss.elastic.co/t/what-determines-the-size-of-ssd-cache-to-install/203129 "2019-10-11T01:12:44Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![neergttocsdivad](https://avatars.discourse-cdn.com/v4/letter/n/43a26b/32.png) [@neergttocsdivad](https://discuss.elastic.co/u/neergttocsdivad)\
**Post date:** [October 11, 2019, 1:12am UTC](https://discuss.elastic.co/t/what-determines-the-size-of-ssd-cache-to-install/203129/1 "2019-10-11T01:12:44Z")

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I am running ES on a Synology NAS VM.

My data and indexes are also stored on the same NAS (same volume).

Bays in the NAS may be allocated to filesystem cache.  
["Elasticsearch heavily relies on the filesystem cache in order to make search fast"](https://www.elastic.co/guide/en/elasticsearch/reference/current/tune-for-search-speed.html)

Is there a way to calculate an optimum size for the SSDs or is there a maximum size beyond which the value decreases or do I install the largest SSDs I can afford?

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**Author:** ![warkolm](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/warkolm/32/39224_2.png) [@warkolm](https://discuss.elastic.co/u/warkolm)\
**Post date:** [October 11, 2019, 2:19am UTC](https://discuss.elastic.co/t/what-determines-the-size-of-ssd-cache-to-install/203129/2 "2019-10-11T02:19:14Z")

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FS cache is a concept managed by the OS, and it's usually done in memory - hence the 50% guideline.

The Synology approach is not directly analogous here, but you could install an SSD and then run Elasticsearch from it entirely.

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**Author:** ![neergttocsdivad](https://avatars.discourse-cdn.com/v4/letter/n/43a26b/32.png) [@neergttocsdivad](https://discuss.elastic.co/u/neergttocsdivad)\
**Post date:** [October 11, 2019, 2:24am UTC](https://discuss.elastic.co/t/what-determines-the-size-of-ssd-cache-to-install/203129/3 "2019-10-11T02:24:28Z")

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thank you for your advice.  
I'm not familiar with a 50% guideline.  
50% of what?

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**Author:** ![warkolm](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/warkolm/32/39224_2.png) [@warkolm](https://discuss.elastic.co/u/warkolm)\
**Post date:** [October 11, 2019, 2:25am UTC](https://discuss.elastic.co/t/what-determines-the-size-of-ssd-cache-to-install/203129/4 "2019-10-11T02:25:22Z")

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50% of total memory - [https://www.elastic.co/guide/en/elasticsearch/reference/current/heap-size.html](https://www.elastic.co/guide/en/elasticsearch/reference/current/heap-size.html)

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<div class="post-metadata">

**Author:** ![neergttocsdivad](https://avatars.discourse-cdn.com/v4/letter/n/43a26b/32.png) [@neergttocsdivad](https://discuss.elastic.co/u/neergttocsdivad)\
**Post date:** [October 11, 2019, 2:45am UTC](https://discuss.elastic.co/t/what-determines-the-size-of-ssd-cache-to-install/203129/5 "2019-10-11T02:45:47Z")

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So, if I put elasticsearch on an SSD volume, would overall performance be better than creating SSD cache or would adding I/O SSD cache speed up the system even more?

I didn't understand your 'heap' suggestion, but, [if I am searching multiple indexes](https://www.elastic.co/guide/en/elasticsearch/reference/current/multi-index.html), totalling around 2Tb, would the 50% be 1Tb (which would obviously be impractical, unaffordable . . .)?

Or, as I have 48Gb of RAM, do you mean 28Gb SSD cache?

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<div class="post-metadata">

**Author:** ![warkolm](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/warkolm/32/39224_2.png) [@warkolm](https://discuss.elastic.co/u/warkolm)\
**Post date:** [October 11, 2019, 2:59am UTC](https://discuss.elastic.co/t/what-determines-the-size-of-ssd-cache-to-install/203129/6 "2019-10-11T02:59:30Z")

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If Elasticsearch runs on an SSD then you will get better performance for sure.

> [@neergttocsdivad](#):
>
> I didn't understand your 'heap' suggestion

Memory being the system RAM available to processes running on said system. Heap being the amount of memory you allocate to the JVM that Elasticsearch starts.

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**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\
**Post date:** [November 8, 2019, 2:59am UTC](https://discuss.elastic.co/t/what-determines-the-size-of-ssd-cache-to-install/203129/7 "2019-11-08T02:59:40Z")

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