Questions: FLUSHALL deletes all the keys but memory used still remains same? How do I release memory used by flushed/purged/deleted keys? I have a key with memory usage of 120 bytes. How much more memory is taken by the TTL of 3600 seconds for this key? Do expired keys also not release memory? Or is it only deleted keys? Premium P1 of 6GB with 1 Shard (2 nodes primary/replica) - Does this mean we have 3GB for read/writes and 3GB is replica? Is this configurable?

Hi @Lodaya, Monica , welcome to Microsoft Q&A forum. Please find the answers to your queries below: FLUSHALL deletes all the keys but memory used still remains same? How do I release memory used by flushed/purged/deleted keys? Response : FLUSHALL will delete all the keys as well as free up the memory. To check it you can go to Console on your Azure Redis Cache and issue ' INFO ' command and check the Memory segment. I tried adding the 3000 keys to Azure Redis Cache and after flushing the keys it released the memory: Before using FLUSHALL: After using FLUSHALL: You can observe the difference here. I have a key with memory usage of 120 bytes. How much more memory is taken by the TTL of 3600 seconds for this key? Response : TTL will not add consume more memory, it will just add the time after which the key will be removed automatically and freeing up the memory. So in your specific case, it is still going to consume 120 bytes irrespective of TTL until key is deleted. Do expired keys also not release memory? Or is it only deleted keys? Response : Expired keys and delete keys both release memory. Only difference between them is Key Expiration automatically removes the keys based on timeout while in Key Deletion we manually have to delete the keys. Premium P1 of 6GB with 1 Shard (2 nodes primary/replica) - Does this mean we have 3GB for read/writes and 3GB is replica? Is this configurable? Response : 6 GB will belong individually to Master and replica. That means both will have 4 GB memory. Please let me know if this helps or else we can discuss further. ---------- If answer helps, please mark it 'Accept Answer'

Azure Redis Cache - How to release memory of deleted keys?

Lodaya, Monica 1

Questions:

FLUSHALL deletes all the keys but memory used still remains same? How do I release memory used by flushed/purged/deleted keys?
I have a key with memory usage of 120 bytes. How much more memory is taken by the TTL of 3600 seconds for this key?
Do expired keys also not release memory? Or is it only deleted keys?
Premium P1 of 6GB with 1 Shard (2 nodes primary/replica) - Does this mean we have 3GB for read/writes and 3GB is replica? Is this configurable?

1 answer

Anurag Sharma 17,606 Reputation points

2021-07-09T10:38:31.233+00:00
Hi @Lodaya, Monica , welcome to Microsoft Q&A forum.

Please find the answers to your queries below:

FLUSHALL deletes all the keys but memory used still remains same? How do I release memory used by flushed/purged/deleted keys?
Response: FLUSHALL will delete all the keys as well as free up the memory. To check it you can go to Console on your Azure Redis Cache and issue 'INFO' command and check the Memory segment. I tried adding the 3000 keys to Azure Redis Cache and after flushing the keys it released the memory:

Before using FLUSHALL:

After using FLUSHALL:

You can observe the difference here.

I have a key with memory usage of 120 bytes. How much more memory is taken by the TTL of 3600 seconds for this key?
Response: TTL will not add consume more memory, it will just add the time after which the key will be removed automatically and freeing up the memory. So in your specific case, it is still going to consume 120 bytes irrespective of TTL until key is deleted.

Do expired keys also not release memory? Or is it only deleted keys?
Response: Expired keys and delete keys both release memory. Only difference between them is Key Expiration automatically removes the keys based on timeout while in Key Deletion we manually have to delete the keys.

Premium P1 of 6GB with 1 Shard (2 nodes primary/replica) - Does this mean we have 3GB for read/writes and 3GB is replica? Is this configurable?
Response: 6 GB will belong individually to Master and replica. That means both will have 4 GB memory.

Please let me know if this helps or else we can discuss further.

----------

If answer helps, please mark it 'Accept Answer'
Please sign in to rate this answer.
Lodaya, Monica 1 Reputation point

2021-07-09T16:59:46.583+00:00

tThanks @AnuragSharma-MSFT for your response. This does help.

On #4 - Premium P1 of 6GB with 1 Shard (2 nodes primary/replica) - Does this mean we have 3GB for read/writes and 3GB is replica? Is this configurable?
Response: 6 GB will belong individually to Master and replica. That means both will have 4 GB memory.
You meant 6GB here, right?

On #1 - Maybe I need more help in understanding those numbers.

Here is my case:
INFO command - Used memory 121MB with 90 keys (all with ttl of 1 hr)
MEMORY STATS command - Bytes per key - 1375925
MEMORY USAGE <key> command - 118-128 bytes

If you look at the stats, memory usage of individual keys is around 120-130 bytes. But bytes per key (used memory/number of keys) is ~1.3MB which is incorrect.

After a day, I have similar number of keys (because of ttl of 1 hr) but used memory is now 146MB.

We have in the past manually inserted and deleted keys from cache.
Also, with ttl set if the number of keys always remains around 90, why is used memory increasing?
I executed Flushall command but used memory is still the same.

Here is the output from Memory stats and Info Memory.

!

Anurag Sharma 17,606 Reputation points

2021-07-12T11:02:37.223+00:00

Thanks for replying back.

Yes you are right there was a typo in point 4. I actually meant 6 GB.

I am checking point 1 with the details provided by you and will get back at the earliest.

Anurag Sharma 17,606 Reputation points

2021-07-13T09:53:59.09+00:00

Hi @Lodaya, Monica , thanks for your patience.

When we create Azure Redis Cache, by default the server allocated the memory (close to 100 MB for P1 premium). Any key that we add, it gets added on top of it, so if we add 10 MB keys, used memory will show 110 MB but in real key memory size is just 10 MBs. Now the commands MEMORY STATS will show the memory of keys plus the default memory allocated. While we go with MEMORY USAGE for key it will show the correct memory allocated key-wise.

Also, as you mentioned the after 24 hours, the used memory grew to 146 MB. Can you please once check if there is any more keys added or what were the number of keys before and after this increase in memory?

Lodaya, Monica 1 Reputation point

2021-07-14T07:27:47.707+00:00

Thanks @AnuragSharma-MSFT for getting back,

Here is the exercise I did at my end. Would appreciate your inputs. Thanks.

Initial Memory - 131.46M Keys - 0 Insert 4million keys (unique arnd 2 million) each of 115 bytes and ttl 1hr Memory - 692M

Ran FLUSHALL - 0 keys and Memory released and back to 131.53M

Inserted 8 million keys data (unique around 4 million) with ttl 1 hr and check if memory released after all keys are expired.

After 1 hr - INFO returns no keys with memory not completely released

Anurag Sharma 17,606 Reputation points

2021-07-15T06:21:02.077+00:00

Hi @Lodaya, Monica , thanks for replying back.

Difference between deleted keys and expired keys is deleted keys claim the memory back instantly while expired keys take sometime to reclaim the memory through deallocator. However in both the cases we cannot access the keys.

If we go through the below article, it mentions the same and it has few ways to reclaim the memory for expired keys:

https://docs.redislabs.com/5.6/ri/memory-optimizations/reclaim-expired-keys-memory-faster/

Also you want to try MEMORY PURGE command which is also used to release memory.

Please let me know if this helps.

----------

if answer helps, you can mark it 'Accept Answer'

Anurag Sharma 17,606 Reputation points

2021-07-20T05:32:36.183+00:00

Hi @Lodaya, Monica , just wanted to follow back on this thread. Please let us know if more information is needed. If answer helped you can mark it as 'Accept Answer'
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Azure Redis Cache - How to release memory of deleted keys?

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