7.6. Using SSD Drives¶

7.6.1. Calculating Write Journal Size¶

Using SSD drives for write journaling can help you reduce write latencies, thus improving the overall cluster performance. If you have multiple chunk servers on a single host, create a separate SSD journal for each CS.

To determine the size of each CS journal on the SSD, follow these guidelines:

1. Find out how many HDDs the SSD can service based on Hardware Requirements. You can also use this formula:

   SSD_SSWS * 0.8 / HDD_SSWS
   

   Where:

   • SSD_SSWS is the sustained sequential write speed of the SSD.

   • HDD_SSWS is the sustained sequential write speed of an HDD (provided that identical HDD models are used as recommended).

   • 0.8 is the approximate percentage of error.

   Note

   The sustained sequential write speed is the average sequential write speed measured over a period of 60 seconds.

2. Reserve 20% of the SSD volume for normal operation, checksum storage, and metadata if needed. Typically, reserve 1GB of SSD space for checksums per each 1TB of HDD space. The rest of these 20% is reserved for emergency needs and also to prevent the SSD from filling up completely because its performance would then degrade.

3. Divide the remaining 80% of the SSD volume by the allowed number of HDDs.

For example, a 512 GB SSD rated at 1500 MB/s SSWS will be able to service 1500 * 0.8 / 150 = 8 HDDs rated at 150 MB/s SSWS. And the journal size for each CS (i.e. HDD) will be (512 - 20%) / 8 = 51 GB.

The following table gives a few examples of SSD models and the amount of HDDs they can service:

7.6.2. Setting Up Chunk Servers with a Journal on SSD¶

To set up a CS that stores a journal on an SSD drive, do the following:

1. Log in to the Node you want to act as a chunk server as root or as a user with root privileges. The Node must have at least one hard disk drive (HDD) and one solid state drive (SSD).

2. Download and install the following RPM packages: vstorage-ctl, vstorage-libs-shared, and vstorage-chunk-server.

   These packages are available in the Virtuozzo remote repository (this repository is automatically configured for your system when you install Virtuozzo) and can be installed with this command:

   # yum install vstorage-chunk-server
   

3. Make sure that cluster discovery is configured for the server. For details, see Configuring Cluster Discovery.

4. Authenticate the server in the cluster, if it is not yet authenticated:

   # vstorage -c stor1 auth-node
   

5. If required, prepare the SSD as described in Preparing Disks for Virtuozzo Storage.

6. Create the chunk server configuration, repository, and the journal, for example:

   # vstorage -c stor1 make-cs -r /vstorage/stor1-cs -j /ssd/stor1/cs1 -s 30720 -T ssd
   

   This command:

   1. Makes the /vstorage/stor1-cs directory on your computer’s hard disk drive and configures it for storing data chunks.

   2. Configures your computer as a chunk server and joins it to the stor1 Virtuozzo Storage cluster.

   3. Creates the journal in the /ssd/stor1/cs1 directory on the SSD drive and allocates 30 GB of disk space to this journal.

   4. Tunes the journal for improved write and read performance on SSD, with best results noticeable on random writes on all-flash clusters.

   Note

   When choosing a directory for the journal and deciding on its size, allocate the required space for the journal and make sure there is 1GB of SSD space per each 1TB of HDD space for checksums.

7. Start the chunk server management service vstorage-csd and configure it to start automatically on the chunk server boot:

   # systemctl start vstorage-csd.target
   # systemctl enable vstorage-csd.target
   

7.6.3. Adding, Destroying, and Configuring CS Journals in Live Storage Clusters¶

# vstorage -c stor1 configure-cs -r <CS_repository_path> \
-a /ssd/stor1-cs1-journal -s 2048 -T ssd

# systemctl status vstorage-csd* | grep -m1 "stor1-cs1"
vstorage-csd.stor1.1027.service - vstorage-csd(/vstorage/stor1-cs1)
# systemctl restart vstorage-csd.stor1.1027.service

# vstorage -c stor1 configure-cs -r <CS_repository_path> -d

# vstorage -c stor1 configure-cs -r <CS_repository_path> -s 4096

7.6.4. Disabling Checksumming¶

Using checksumming, you can provide better reliability and integrity of all data in the cluster. When checksumming is enabled, Virtuozzo Storage generates checksums each time some data in the cluster is modified. When this data is then read, the checksum is computed once more and compared with the already existing value.

By default, data checksumming is automatically enabled for newly created chunk servers that use journaling. If necessary, you can disable this functionality using the -S option when you set up a chunk server, for example:

# vstorage -c stor1 make-cs -r /vstorage/stor1-cs -j /ssd/stor1/cs1 -s 30720 -S

7.6.5. Configuring Data Scrubbing¶

Data scrubbing is the process of checking data chunks for durability and verifying their contents for readability and correctness. By default, Virtuozzo Storage is set to examine two data chunks per minute on each chunk server in the cluster. If necessary, you can configure this number using the vstorage utility, for example:

# vstorage -c stor1 set-config mds.wd.verify_chunks=3

This command sets the number of chunks to be examined on each chunk server in the stor1 cluster to 3.