Document new behaviour of cache limiter

cmd/copy.go

@@ -1973,7 +1973,8 @@ func init() {
 	cpCmd.PersistentFlags().StringVar(&raw.excludeBlobType, "exclude-blob-type", "", "Optionally specifies the type of blob (BlockBlob/ PageBlob/ AppendBlob) to exclude when copying blobs from the container "+
 		"or the account. Use of this flag is not applicable for copying data from non azure-service to service. More than one blob should be separated by ';'. ")
 	// options change how the transfers are performed
-	cpCmd.PersistentFlags().Float64Var(&raw.blockSizeMB, "block-size-mb", 0, "Use this block size (specified in MiB) when uploading to Azure Storage, and downloading from Azure Storage. The default value is automatically calculated based on file size. Decimal fractions are allowed (For example: 0.25).")
+	cpCmd.PersistentFlags().Float64Var(&raw.blockSizeMB, "block-size-mb", 0, "Use this block size (specified in MiB) when uploading to Azure Storage, and downloading from Azure Storage. The default value is automatically calculated based on file size. Decimal fractions are allowed (For example: 0.25)."+
+	" When uploading or downloading, maximum allowed block size is 0.75 * AZCOPY_BUFFER_GB. Please refer https://learn.microsoft.com/en-us/azure/storage/common/storage-use-azcopy-optimize#optimize-memory-use.")
 	cpCmd.PersistentFlags().StringVar(&raw.blobType, "blob-type", "Detect", "Defines the type of blob at the destination. This is used for uploading blobs and when copying between accounts (default 'Detect'). Valid values include 'Detect', 'BlockBlob', 'PageBlob', and 'AppendBlob'. "+
 		"When copying between accounts, a value of 'Detect' causes AzCopy to use the type of source blob to determine the type of the destination blob. When uploading a file, 'Detect' determines if the file is a VHD or a VHDX file based on the file extension. If the file is either a VHD or VHDX file, AzCopy treats the file as a page blob.")
 	cpCmd.PersistentFlags().StringVar(&raw.blockBlobTier, "block-blob-tier", "None", "upload block blob to Azure Storage using this blob tier. (default 'None'). Valid options are Hot, Cold, Cool, Archive")