通过慢速网络下载大文件时出现 StorageException

Question

我正在使用 NuGet 包WindowsAzure.Storage 4.2.1 版。

以下代码尝试从位于远程数据中心的存储容器下载 Blob。

try
{
    var blobRequestOptions = new BlobRequestOptions
    {
        RetryPolicy = new ExponentialRetry(TimeSpan.FromSeconds(5), 3),
        MaximumExecutionTime = TimeSpan.FromMinutes(60),
        ServerTimeout = TimeSpan.FromMinutes(60)
    };
    using (var fileStream = File.Create(localPath))
    {
        blockBlob.DownloadToStream(fileStream, null, blobRequestOptions);
    }
}
catch (Exception e)
{
    Console.WriteLine(e.Message);
}

但是，有时它会下载约 10 分钟，然后引发以下异常：

Unhandled Exception: Microsoft.WindowsAzure.Storage.StorageException: The client could not finish the operation within specified timeout. ---> System.TimeoutException: The client could not finish the operation within specified timeout.
--- End of inner exception stack trace ---
at Microsoft.WindowsAzure.Storage.Core.Util.StorageAsyncResult`1.End()
at Microsoft.WindowsAzure.Storage.Blob.CloudBlockBlob.EndUploadText(IAsyncResult asyncResult)
at Microsoft.WindowsAzure.Storage.Core.Util.AsyncExtensions.<>c__DisplayClass4.b__3(IAsyncResult ar)
--- End of stack trace from previous location where exception was thrown ---

我该如何解决这个问题？

Answer 1

请尝试此代码。基本上它的作用是首先创建一个空文件，然后使用DownloadRangeToStream 方法以 1 MB 块读取 blob 的数据。随着块的下载，它会将其附加到文件中。

    private static void DownloadLargeFile()
    {
        var account = new CloudStorageAccount(new StorageCredentials(accountName, accountKey), true);
        var blobClient = account.CreateCloudBlobClient();
        var container = blobClient.GetContainerReference("container-name");
        var file = "my-very-large-file-name";
        var blob = container.GetBlockBlobReference(file);
        //First fetch the size of the blob. We use this to create an empty file with size = blob's size
        blob.FetchAttributes();
        var blobSize = blob.Properties.Length;
        long blockSize = (1 * 1024 * 1024);//1 MB chunk;
        blockSize = Math.Min(blobSize, blockSize);
        //Create an empty file of blob size
        using (FileStream fs = new FileStream(file, FileMode.Create))//Create empty file.
        {
            fs.SetLength(blobSize);//Set its size
        }
        var blobRequestOptions = new BlobRequestOptions
        {
            RetryPolicy = new ExponentialRetry(TimeSpan.FromSeconds(5), 3),
            MaximumExecutionTime = TimeSpan.FromMinutes(60),
            ServerTimeout = TimeSpan.FromMinutes(60)
        };
        long currentPointer = 0;
        long bytesRemaining = blobSize;
        do
        {
            var bytesToFetch = Math.Min(blockSize, bytesRemaining);
            using (MemoryStream ms = new MemoryStream())
            {
                //Download range (by default 1 MB)
                blob.DownloadRangeToStream(ms, currentPointer, bytesToFetch, null, blobRequestOptions);
                ms.Position = 0;
                var contents = ms.ToArray();
                using (var fs = new FileStream(file, FileMode.Open))//Open that file
                {
                    fs.Position = currentPointer;//Move the cursor to the end of file.
                    fs.Write(contents, 0, contents.Length);//Write the contents to the end of file.
                }
                currentPointer += contents.Length;//Update pointer
                bytesRemaining -= contents.Length;//Update bytes to fetch
            }
        }
        while (bytesRemaining > 0);
    }

我已在一个小文件上尝试过此代码（互联网连接不佳：P）。因此，在尝试使用 500 MB 文件之前，请先在一个小文件（比如 5 - 10 MB 左右）上进行尝试。 HTH。