Snippet Name: Check_temp_space_MySQL
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Created Date: 2025-06-16 13:12:43
Last Modified Date: 2025-05-31 10:32:16
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Title: Check temp space MySQL
Author: user
in MySQL, how can i check the temp space? i got an error in an app "MySQL: The table '/rdsdbdata/tmp/#sql136_a15da_3' is full"
Author: ChatGPT
The error message you're seeing ???
`"MySQL: The table '/rdsdbdata/tmp/#sql136_a15da_3' is full"` ???
means MySQL ran out of temporary space while performing a query, typically due to insufficient **disk space** on the partition used for **temporary tables**.
### ??? How to check temp space usage and limits in MySQL:
#### 1. **Check temp table location**
Use this query to confirm the location where MySQL writes temp tables:
```sql
SHOW VARIABLES LIKE 'tmpdir';
```
Common values might be:
- `/tmp`
- `/rdsdbdata/tmp` (as in your case ??? AWS RDS default)
#### 2. **Check disk space on the temp partition (OS-level)**
If you???re on **self-hosted MySQL**, use:
```bash
df -h /rdsdbdata/tmp
```
But if you're on **AWS RDS**, you can???t run `df`, so instead:
#### 3. **Check RDS Free Storage Space**
For AWS RDS, use this SQL query to view free space:
```sql
SELECT
ROUND(SUM(free_space) / 1024 / 1024 / 1024, 2) AS FreeSpaceGB
FROM
information_schema.files
WHERE
tablespace_name = 'innodb_temporary';
```
You can also look at the CloudWatch metric:
- `FreeStorageSpace` ??? for the entire RDS instance
#### 4. **Check current tmp table usage**
```sql
SHOW GLOBAL STATUS LIKE 'Created_tmp%';
```
These values give insight into how many temp tables are being created, and whether they spill to disk:
- `Created_tmp_disk_tables` (bad if high)
- `Created_tmp_tables` (total)
#### 5. **See max size for temp tables in memory**
If a temp table exceeds this limit, it spills to disk:
```sql
SHOW VARIABLES LIKE 'tmp_table_size';
SHOW VARIABLES LIKE 'max_heap_table_size';
```
Make sure both are **increased together** (they work together):
```sql
SET GLOBAL tmp_table_size = 512*1024*1024;
SET GLOBAL max_heap_table_size = 512*1024*1024;
```
(*Only works until restart; for permanence, change your MySQL config*)
---
### ???? Fix Options
- **Add more disk** to your RDS instance or change instance class with more storage.
- **Increase `tmp_table_size` and `max_heap_table_size`** to reduce disk usage.
- **Optimize queries** creating large temp tables (e.g., avoid large `GROUP BY`, `ORDER BY` on unindexed columns).
- **Use monitoring** (CloudWatch, Performance Insights) to catch spikes early.
Would you like help identifying which query is likely causing this temp space issue?
Author: user
both tmp_table_size and max_heap_table_size seem to be teh same value, which i think is normal, right? 16777216
Author: ChatGPT
Yes, you're absolutely right ??? having both `tmp_table_size` and `max_heap_table_size` set to the **same value** is standard and **required for consistency**. The default value of:
```
16777216 bytes = 16 MB
```
...is quite **low** for many modern workloads. This means **any temporary table exceeding 16 MB in RAM** will be written to disk ??? and in your case, that disk is running out of space, triggering the error:
> `"The table '/rdsdbdata/tmp/#sql...' is full"`
---
### ???? Recommendation: Increase These Limits
If your workload can support it, consider increasing both variables **together** to something more generous ??? for example:
```sql
SET GLOBAL tmp_table_size = 128 * 1024 * 1024; -- 128MB
SET GLOBAL max_heap_table_size = 128 * 1024 * 1024; -- 128MB
```
Or even:
```sql
SET GLOBAL tmp_table_size = 512 * 1024 * 1024;
SET GLOBAL max_heap_table_size = 512 * 1024 * 1024;
```
> ?????? These changes last **only until the next MySQL restart**, unless persisted in configuration (on non-RDS setups).
> On **RDS**, you must use a **parameter group** to make them permanent:
- Go to the RDS console ??? Parameter Groups
- Modify or create a custom group
- Set both:
- `tmp_table_size = 134217728` (128MB)
- `max_heap_table_size = 134217728` (128MB)
- Attach the parameter group to your RDS instance and reboot
---
### ???? Bonus: Why both?
> MySQL limits in-memory temporary tables by **whichever is lower** of the two variables:
```text
Actual limit = MIN(tmp_table_size, max_heap_table_size)
```
So even if `tmp_table_size` is 128MB, but `max_heap_table_size` is still 16MB, memory temp tables will spill at 16MB.
---
Would you like help crafting a safe script to increase and monitor these settings on RDS?
Author: user
i got this error, referencing a database that doe snto exist. is this related to temp? Invalid custom query 'ADS2 Top queries': [Unknown database 'qsdb_c8ce7c74e4bb47e6b1e5c196ad743b38']
Author: ChatGPT
That error is **not related to temp space**, but instead points to an **invalid or deleted database reference** in a **custom query**, likely in a monitoring or analytics tool (such as AWS Performance Insights, AppDynamics, Datadog, or a similar SQL dashboard).
### ???? Error Breakdown:
```
Invalid custom query 'ADS2 Top queries':
[Unknown database 'qsdb_c8ce7c74e4bb47e6b1e5c196ad743b38']
```
### ???? What this likely means:
- A **custom query** (named `ADS2 Top queries`) is attempting to run SQL against a database named `qsdb_c8ce7c74e4bb47e6b1e5c196ad743b38`
- That database **no longer exists**, was **never created**, or has been **renamed or dropped**
- The application or dashboard hasn???t been updated to reflect that change
### ??? What to do next:
1. **Search your monitoring tool** (e.g. ADS2, AppDynamics, etc.) for a saved query or configuration called `'Top queries'`.
2. **Edit the custom query** to:
- Reference the correct existing database
- Or remove the query if it's no longer needed
3. If you're unsure which tool is running this query:
- Check your MySQL general log or slow query log (if enabled)
- Check for scheduled jobs or external tools querying the instance
---
### ??? Confirm it's not a temp table issue:
Temporary tables use file-based names like:
```
#sql123_456.tmp or #sqlabc_xyz.MYD
```
Whereas the above is clearly a **named schema** (not temporary), which failed to resolve.
Would you like help identifying where this query might be coming from (e.g. log analysis or app config hints)?
Author: user
the free space is there a limit on the amount of space dedicated to temp?
Author: ChatGPT
Great question. In **MySQL**, especially on **AWS RDS**, there is **no hardcoded limit on the size of temporary tables** ??? **except for the physical storage available** on the disk or partition used for temporary files (typically `tmpdir`, which for RDS is often `/rdsdbdata/tmp`).
However, there are important caveats:
---
### ??? **What governs temp space usage**
1. **Available Disk Space (Primary Limit)**
- MySQL uses the storage allocated to your instance (or partition, on non-RDS) for temp tables.
- When this is exhausted, you get the error:
`The table '/rdsdbdata/tmp/#sql...' is full`
2. **Filesystem Limits**
- If your `tmpdir` is on a separate volume or partition (common in some setups), then only that partition???s free space matters.
3. **Engine-specific behavior**
- **InnoDB temporary tables** (used more in recent versions) are stored in the `innodb_temp` tablespace.
- You can check that space with:
```sql
SELECT * FROM information_schema.innodb_temp_table_info;
```
4. **MySQL Variables ??? but not actual hard limits:**
- `tmp_table_size` and `max_heap_table_size` **only affect in-memory** temporary tables.
- Once memory limits are exceeded, MySQL spills to disk.
- No variable limits how much *disk* space temp tables can consume ??? only the OS/filesystem does.
---
### ???? In AWS RDS, Important Considerations:
- RDS uses **EBS volumes** for storage, and **temp files are stored on the same volume as your data** unless explicitly separated.
- **Temp space is shared** with other things: InnoDB data, transaction logs, binary logs, undo logs, etc.
To check total disk usage in RDS:
```sql
SELECT table_schema AS DBName,
SUM(data_length + index_length) / 1024 / 1024 AS SizeMB
FROM information_schema.tables
GROUP BY table_schema;
```
Then compare that to **CloudWatch ??? `FreeStorageSpace` metric**.
---
### ???? Ways to Avoid This Issue
1. **Use larger instance storage (scale vertically or increase EBS size)**
2. **Refactor queries** causing large temp table creation (e.g., `ORDER BY`, `GROUP BY`, joins on unindexed fields)
3. **Increase memory temp limits** (as discussed: `tmp_table_size` and `max_heap_table_size`)
4. **Use monitoring** to track temp disk usage over time
---
Would you like help writing a script that tracks current temp table usage and warns when disk is nearly full?
Author: user
you mentioned this: Available Disk Space (Primary Limit)
MySQL uses the storage allocated to your instance (or partition, on non-RDS) for temp tables.
When this is exhausted, you get the error:
The table '/rdsdbdata/tmp/#sql...' is full, but i have 70GB of disk, and 8gb of ram, i don't think i hit a limit, but i got an error
Author: ChatGPT
Thanks, Lowell ??? your observation is right on target: if you have **70 GB of disk** and **8 GB of RAM**, and still hit this error:
```
The table '/rdsdbdata/tmp/#sql...' is full
```
...then **you almost certainly did not run out of total disk space**, **but** you may have hit **one of these less obvious limits**:
---
### ???? Less Obvious Causes of "Temp Table Is Full" (Despite Free Space)
#### 1. ??? **Filesystem or Partition Full**
Even with 70 GB free *overall*, check if `/rdsdbdata/tmp` is on a **separate partition** (common in some RDS and Linux configurations).
If it is, that partition may be full even when the overall disk is not.
> On RDS, you can???t SSH to run `df -h`, but you can:
- Check `FreeStorageSpace` in CloudWatch (it???s per EBS volume)
- Check `information_schema.innodb_temporary` or error logs for usage spikes
---
#### 2. ???? **Internal Table Format Limitations**
In older versions of MySQL or **when using `MyISAM` temporary tables**, you can hit **individual file size limits** even on large disks:
- **MyISAM** has a 256 TB theoretical file size limit, but this is reduced by:
- Filesystem limits (e.g., 2GB or 4GB if mounted with certain options)
- Per-table size limits in MySQL (if `max_heap_table_size` is low)
- AppArmor / SELinux or AWS sandboxing in managed environments (yes, even in RDS)
---
#### 3. ???? **RDS Storage I/O or Quota Constraints**
Even with disk free, RDS can **throttle** large bursts of temp space usage:
- If you're hitting **I/O throughput** limits (e.g., burstable volumes with low baseline performance)
- Or you're consuming **ephemeral temp disk** too quickly
- Or **AWS internal quotas** on file handle usage or temp file size
Check:
- CloudWatch metrics: `FreeableMemory`, `ReadIOPS`, `WriteIOPS`, `DiskQueueDepth`
---
#### 4. ?????? **Implicit Temp Table Overflows**
If a **large `GROUP BY`, `ORDER BY`, or JOIN** causes a temp table that:
- Can't fit in memory (`tmp_table_size`/`max_heap_table_size`), and
- Is too large for disk spillover in time or space
Then MySQL may abort early with this error, even when disk seems free.
---
#### 5. ??????? **InnoDB Temp Tablespace Corruption or Saturation**
If you???re using **InnoDB temp tables** (most likely on recent MySQL versions):
Run:
```sql
SELECT * FROM information_schema.innodb_temp_table_info;
```
You may discover:
- Large orphaned temp tables
- Long-running sessions holding onto them
---
### ??? Suggested Steps
1. **Query InnoDB temp usage:**
```sql
SELECT * FROM information_schema.innodb_temp_table_info;
```
2. **Query tmp table creation counts:**
```sql
SHOW GLOBAL STATUS LIKE 'Created_tmp%';
```
3. **Confirm `tmpdir` and temp table settings:**
```sql
SHOW VARIABLES LIKE 'tmpdir';
SHOW VARIABLES LIKE 'tmp_table_size';
SHOW VARIABLES LIKE 'max_heap_table_size';
```
4. **Check RDS CloudWatch:**
- `FreeStorageSpace`
- `DiskQueueDepth`
- `ReadIOPS` / `WriteIOPS`
---
### ???? If You're Still Stuck
We can build a temporary **diagnostic SQL script** that:
- Lists temp table counts by session
- Checks current disk-consuming processes
- Warns about spills to disk
Would you like me to generate such a script tailored to your environment (e.g., AWS RDS MySQL)?