Managing Disk Space for Impala Data
Although Impala typically works with many large files in an HDFS storage system with plenty of capacity, there are times when you might perform some file cleanup to reclaim space, or advise developers on techniques to minimize space consumption and file duplication.
Use compact binary file formats where practical. Numeric and time-based data in particular can be stored in more compact form in binary data files. Depending on the file format, various compression and encoding features can reduce file size even further. You can specify the
STORED ASclause as part of the
CREATE TABLEstatement, or
ALTER TABLEwith the
SET FILEFORMATclause for an existing table or partition within a partitioned table. See How Impala Works with Hadoop File Formats for details about file formats, especially Using the Parquet File Format with Impala Tables. See CREATE TABLE Statement and ALTER TABLE Statement for syntax details.
You manage underlying data files differently depending on whether the corresponding Impala table is defined as an internal or external table:
DESCRIBE FORMATTEDstatement to check if a particular table is internal (managed by Impala) or external, and to see the physical location of the data files in HDFS. See DESCRIBE Statement for details.
For Impala-managed ("internal") tables, use
DROP TABLEstatements to remove data files. See DROP TABLE Statement for details.
For tables not managed by Impala ("external" tables), use appropriate
HDFS-related commands such as
hdfs dfs, or
distcp, to create, move, copy, or delete files within HDFS directories that are accessible by the
impalauser. Issue a
REFRESH table_namestatement after adding or removing any files from the data directory of an external table. See REFRESH Statement for details.
- Use external tables to reference HDFS data files in their original location. With this technique, you avoid copying the files, and you can map more than one Impala table to the same set of data files. When you drop the Impala table, the data files are left undisturbed. See External Tables for details.
LOAD DATAstatement to move HDFS files into the data directory for an Impala table from inside Impala, without the need to specify the HDFS path of the destination directory. This technique works for both internal and external tables. See LOAD DATA Statement for details.
- Use the
Make sure that the HDFS trashcan is configured correctly. When you remove files from HDFS, the space might not be reclaimed for use by other files until sometime later, when the trashcan is emptied. See DROP TABLE Statement for details. See User Account Requirements for permissions needed for the HDFS trashcan to operate correctly.
Drop all tables in a database before dropping the database itself. See DROP DATABASE Statement for details.
Clean up temporary files after failed
INSERTstatements. If an
INSERTstatement encounters an error, and you see a directory named .impala_insert_staging or _impala_insert_staging left behind in the data directory for the table, it might contain temporary data files taking up space in HDFS. You might be able to salvage these data files, for example if they are complete but could not be moved into place due to a permission error. Or, you might delete those files through commands such as
hdfs dfs, to reclaim space before re-trying the
DESCRIBE FORMATTED table_nameto see the HDFS path where you can check for temporary files.
If you use the Amazon Simple Storage Service (S3) as a place to offload data to reduce the volume of local storage, Impala 2.2.0 and higher can query the data directly from S3. See Using Impala with Amazon S3 Object Store for details.
Configuring Scratch Space for Spilling to DiskImpala uses intermediate files during large sort, join, aggregation, or analytic function operations The files are removed when the operation finishes. You can specify locations of the intermediate files by starting the impalad daemon with the
‑‑scratch_dirs="path_to_directory"configuration option. By default, intermediate files are stored in the directory /tmp/impala-scratch.
- You can specify a single directory or a comma-separated list of directories.
You can specify an optional a capacity quota per scratch directory using the colon
(:) as the delimiter.
The capacity quota of
0is the same as no quota for the directory.
- The scratch directories must be on the local filesystem, not in HDFS.
- You might specify different directory paths for different hosts, depending on the capacity and speed of the available storage devices.
If there is less than 1 GB free on the filesystem where that directory resides, Impala still runs, but writes a warning message to its log.
Impala successfully starts (with a warning written to the log) if it cannot create or read and write files in one of the scratch directories.
||Use /dir1 and /dir2 as scratch directories with no capacity quota.|
||Use /dir1 and /dir2 as scratch directories with no capacity quota on /dir1 and the 25GB quota on /dir2.|
||Use /dir1 and /dir2 as scratch directories with the capacity quota of 5MB on /dir1 and no quota on /dir2.|
||Use /dir1 and /dir2 as scratch directories with no capacity quota.|
Allocation from a scratch directory will fail if the specified limit for the directory is exceeded.
If Impala encounters an error reading or writing files in a scratch directory during a query, Impala logs the error, and the query fails.
Priority Based Scratch Directory Selection
The location of the intermediate files are configured by starting the impalad daemon with the flag ‑‑scratch_dirs="path_to_directory". Currently this startup flag uses the configured scratch directories in a round robin fashion. Automatic selection of scratch directories in a round robin fashion may not always be ideal in every situation since these directories could come from different classes of storage system volumes having different performance characteristics (SSD vs HDD, local storage vs network attached storage, etc.). To optimize your workload, you have an option to configure the priority of the scratch directories based on your storage system configuration.
The scratch directories will be selected for spilling based on how you configure the priorities of the directories and if you provide the same priority for multiple directories then the directories will be selected in a round robin fashion.
/dir1 /dir1:200GB /dir1:200GB:
In the example below, dir1 will be used as a spill victim until it is full and then dir2, dir3, and dir4 will be used in a round robin fashion.
‑‑scratch_dirs="/dir1:200GB:0, /dir2:1024GB:1, /dir3:1024GB:1, /dir4:1024GB:1"
Increasing Scratch Capacity
You can compress the data spilled to disk to increase the effective scratch capacity. You
typically more than double capacity using compression and reduce spilling to disk. Use the
--disk_spill_compression_codec and –-disk_spill_punch_holes startup options. The
--disk_spill_compression_codec takes any value supported by the COMPRESSION_CODEC query
option. The value is not case-sensitive. A value of
LZ4 is recommended (default is NONE).
If you set
--disk_spill_compression_codec to a value other than
NONE, you must set
--disk_spill_punch_holes to true.
The hole punching feature supported by many filesystems is used to reclaim space in scratch files during execution of a query that spills to disk. This results in lower scratch space requirements in many cases, especially when combined with disk spill compression. When this option is not enabled, scratch space is still recycled by a query, but less effectively in many cases.
You can specify a compression level for
ZSTD only. For example:
Compression levels from 1 up to 22 (default 3) are supported for
The lower the compression level, the faster the speed at the cost of compression ratio.