![]() ![]() There are specific Azure VM series (M-series at current) that support something known as “Write Accelerator”. Storage tech vendors try to mitigate this by putting batteries into the storage appliances, but since the write cache in Azure is at the physical host level, there’s just no guarantee that when the VM O/S thinks the write operation has committed to disk, that it has actually been written to disk. It’s worth noting that this cache “issue” has always been true of every caching technology ever created, on which databases run. This is not good for databases, especially if the issue affects the transaction/redo log area. The reason for not using a write cache every time, is because an issue at the physical host level, affecting the cache, could cause the application (e.g database) to think it has committed data, when it actually isn’t written to disk. The above settings for HANA have been taken from the updated guide here: which is designed to meet the KPIs mentioned in SAP note 2762990. It is envisaged that using ASE 16.0 with SRS/HADR you would have a separate data disk for the Replication Server data (I’ll talk about this in another post). The use of write caching on the ASE home is optional, you could choose ReadOnly, it would help protect the ASE config file in a very specific scenario. The above settings for SAP ASE have been obtained from SAP note 2367194 (SQL Server is same as ASE) and from the general deployment guide here: There are specific recommendations for Azure disk cache settings, especially when running SAP and especially when running databases like SAP ASE or SAP HANA. Cache Setting: ReadWriteīy specifying “ReadWrite” as the cache setting, a write operation at the VM O/S level is confirmed as completed once the data is written to the cache on the underlying physical machine.Īll read operations for data not already in the VM O/S file system cache, will be read from the read cache on the underlying physical machine, before being read from the storage service.Īny Recommendations for Disk Cache Settings? Cache Setting: ReadOnlyīy specifying “ReadOnly” as the cache setting, a write operation at the VM O/S level is confirmed as completed once the data is written to the storage service.Īll read operations for data not already in the VM O/S file system cache, will be read from the read cache on the underlying physical machine, before being read from the storage service. By specifying “None” as the cache setting, no caching is used and a write operation at the VM O/S level is confirmed as completed once the data is written to the storage service.Īll read operations for data not already in the VM O/S file system cache, will be read from the storage service.
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